simplemagnets-1.0.10a-mc1.16.4

    Simple Magnets (simplemagnets) has failed to load correctly

?7java.lang.NoClassDefFoundError: net/minecraft/client/util/ITooltipFlag
]
at net.minecraftforge.fml.ModLoader.waitForTransition(ModLoader.java:263) ~[forge:?] {re:classloading}
at net.minecraftforge.fml.ModLoader.dispatchAndHandleError(ModLoader.java:230) ~[forge:?] {re:classloading}
at net.minecraftforge.fml.ModLoader.gatherAndInitializeMods(ModLoader.java:188) ~[forge:?] {re:classloading}
at net.minecraftforge.fml.server.ServerModLoader.load(ServerModLoader.java:44) ~[forge:?] {re:classloading}
at net.minecraft.server.Main.main(Main.java:95) ~[?:?] {re:classloading}
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_261] {}
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_261] {}
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_261] {}
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_261] {}
at net.minecraftforge.fml.loading.FMLServerLaunchProvider.lambda$launchService$0(FMLServerLaunchProvider.java:51) ~[forge-1.16.4-35.1.36.jar:35.1] {}
at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-8.0.6.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-8.0.6.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-8.0.6.jar:?] {}
at cpw.mods.modlauncher.Launcher.run(Launcher.java:82) [modlauncher-8.0.6.jar:?] {re:classloading}
at cpw.mods.modlauncher.Launcher.main(Launcher.java:66) [modlauncher-8.0.6.jar:?] {re:classloading}
at net.minecraftforge.server.ServerMain$Runner.runLauncher(ServerMain.java:63) [forge-1.16.4-35.1.36.jar:?] {}
at net.minecraftforge.server.ServerMain$Runner.access$100(ServerMain.java:60) [forge-1.16.4-35.1.36.jar:?] {}
at net.minecraftforge.server.ServerMain.main(ServerMain.java:57) [forge-1.16.4-35.1.36.jar:?] {}

Was this tested against multiplayer?

Seems I forgot to copy the OnlyIn(Dist.Client) annotation 🤦‍♂️
I fixed it now in version 1.0.11.

Thanks for reporting the issue!