When I try launching with version 5.2.1-all it says its not a valid mod file.
Additionally, in the log it says that it is missing a required field in the dependency:

net.minecraftforge.fml.loading.moddiscovery.InvalidModFileException: Missing required field mandatory in dependency (mffs-5.2.1-all.jar)
at net.minecraftforge.fml.loading.moddiscovery.ModInfo$ModVersion.lambda$new$1(ModInfo.java:217) ~[fmlloader-1.20.4-49.0.3.jar:?]
at java.util.Optional.orElseThrow(Optional.java:403) ~[?:?]
at net.minecraftforge.fml.loading.moddiscovery.ModInfo$ModVersion.(ModInfo.java:217) ~[fmlloader-1.20.4-49.0.3.jar:?]
at net.minecraftforge.fml.loading.moddiscovery.ModInfo.lambda$new$7(ModInfo.java:100) ~[fmlloader-1.20.4-49.0.3.jar:?]
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197) ~[?:?]
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1625) ~[?:?]
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509) ~[?:?]
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499) ~[?:?]
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:575) ~[?:?]
at java.util.stream.AbstractPipeline.evaluateToArrayNode(AbstractPipeline.java:260) ~[?:?]
at java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:616) ~[?:?]
at java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:622) ~[?:?]
at java.util.stream.ReferencePipeline.toList(ReferencePipeline.java:627) ~[?:?]
at net.minecraftforge.fml.loading.moddiscovery.ModInfo.(ModInfo.java:101) ~[fmlloader-1.20.4-49.0.3.jar:?]
at net.minecraftforge.fml.loading.moddiscovery.ModFileInfo.lambda$new$2(ModFileInfo.java:85) ~[fmlloader-1.20.4-49.0.3.jar:?]
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197) ~[?:?]
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1625) ~[?:?]
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509) ~[?:?]
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499) ~[?:?]
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:575) ~[?:?]
at java.util.stream.AbstractPipeline.evaluateToArrayNode(AbstractPipeline.java:260) ~[?:?]
at java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:616) ~[?:?]
at java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:622) ~[?:?]
at java.util.stream.ReferencePipeline.toList(ReferencePipeline.java:627) ~[?:?]
at net.minecraftforge.fml.loading.moddiscovery.ModFileInfo.(ModFileInfo.java:86) ~[fmlloader-1.20.4-49.0.3.jar:?]
at net.minecraftforge.fml.loading.moddiscovery.ModFileParser.modsTomlParser(ModFileParser.java:46) ~[fmlloader-1.20.4-49.0.3.jar:?]
at net.minecraftforge.fml.loading.moddiscovery.ModFileParser.readModList(ModFileParser.java:30) ~[fmlloader-1.20.4-49.0.3.jar:?]
at net.minecraftforge.fml.loading.moddiscovery.ModFile.(ModFile.java:71) ~[fmlloader-1.20.4-49.0.3.jar:?]
at net.minecraftforge.fml.loading.moddiscovery.ModFile.(ModFile.java:60) ~[fmlloader-1.20.4-49.0.3.jar:?]
at net.minecraftforge.fml.loading.moddiscovery.AbstractModProvider.createMod(AbstractModProvider.java:52) ~[fmlloader-1.20.4-49.0.3.jar:?]
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197) ~[?:?]
at java.util.ArrayList.forEach(ArrayList.java:1511) ~[?:?]
at java.util.stream.SortedOps$RefSortingSink.end(SortedOps.java:395) ~[?:?]
at java.util.stream.Sink$ChainedReference.end(Sink.java:258) ~[?:?]
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:510) ~[?:?]
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499) ~[?:?]
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:575) ~[?:?]
at java.util.stream.AbstractPipeline.evaluateToArrayNode(AbstractPipeline.java:260) ~[?:?]
at java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:616) ~[?:?]
at java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:622) ~[?:?]
at java.util.stream.ReferencePipeline.toList(ReferencePipeline.java:627) ~[?:?]
at net.minecraftforge.fml.loading.moddiscovery.ModsFolderLocator.scanMods(ModsFolderLocator.java:49) ~[fmlloader-1.20.4-49.0.3.jar:?]
at net.minecraftforge.fml.loading.moddiscovery.ModDiscoverer.discoverMods(ModDiscoverer.java:78) ~[fmlloader-1.20.4-49.0.3.jar:?]
at net.minecraftforge.fml.loading.FMLLoader.beginModScan(FMLLoader.java:154) ~[fmlloader-1.20.4-49.0.3.jar:49.0.3]
at net.minecraftforge.fml.loading.FMLServiceProvider.beginScanning(FMLServiceProvider.java:58) ~[fmlloader-1.20.4-49.0.3.jar:49.0.3]
at cpw.mods.modlauncher.TransformationServiceDecorator.runScan(TransformationServiceDecorator.java:98) ~[modlauncher-10.1.2.jar:?]
at cpw.mods.modlauncher.TransformationServicesHandler.lambda$runScanningTransformationServices$7(TransformationServicesHandler.java:96) ~[modlauncher-10.1.2.jar:?]
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197) ~[?:?]
at java.util.HashMap$ValueSpliterator.forEachRemaining(HashMap.java:1779) ~[?:?]
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509) ~[?:?]
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499) ~[?:?]
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:575) ~[?:?]
at java.util.stream.AbstractPipeline.evaluateToArrayNode(AbstractPipeline.java:260) ~[?:?]
at java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:616) ~[?:?]
at java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:622) ~[?:?]
at java.util.stream.ReferencePipeline.toList(ReferencePipeline.java:627) ~[?:?]
at cpw.mods.modlauncher.TransformationServicesHandler.runScanningTransformationServices(TransformationServicesHandler.java:98) ~[modlauncher-10.1.2.jar:?]
at cpw.mods.modlauncher.TransformationServicesHandler.initializeTransformationServices(TransformationServicesHandler.java:49) ~[modlauncher-10.1.2.jar:?]
at cpw.mods.modlauncher.Launcher.run(Launcher.java:86) ~[modlauncher-10.1.2.jar:?]
at cpw.mods.modlauncher.Launcher.main(Launcher.java:73) ~[modlauncher-10.1.2.jar:?]
at cpw.mods.modlauncher.BootstrapEntry.main(BootstrapEntry.java:17) ~[modlauncher-10.1.2.jar:?]
at net.minecraftforge.bootstrap.Bootstrap.moduleMain(Bootstrap.java:97) ~[bootstrap-2.0.0.jar!/:?]
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:?]
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[?:?]
at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?]
at java.lang.reflect.Method.invoke(Method.java:568) ~[?:?]
at net.minecraftforge.bootstrap.Bootstrap.start(Bootstrap.java:48) ~[bootstrap-2.0.0.jar!/:?]
at net.minecraftforge.bootstrap.ForgeBootstrap.main(ForgeBootstrap.java:18) ~[bootstrap-2.0.0.jar!/:?]`

The published jar's modloader has been wrongly set to Forge instead of NeoForge, sorry. I've updated it to point to NeoForge instead.