[14:23:17] [main/FATAL]: Failed to start the minecraft server
java.lang.RuntimeException: Could not execute entrypoint stage 'main' due to errors, provided by 'firstperson'!
at Not Enough Crashes deobfuscated stack trace.(1.16.4+build.7) ~[?:?]
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke0(EntrypointUtils.java:53) ~[fabric-server-launch.jar:?]
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke(EntrypointUtils.java:36) ~[fabric-server-launch.jar:?]
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointServer.start(EntrypointServer.java:32) ~[fabric-server-launch.jar:?]
at net.minecraft.server.Main.main(Main:92) [intermediary-server.jar:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_275]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_275]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_275]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_275]
at net.fabricmc.loader.game.MinecraftGameProvider.launch(MinecraftGameProvider.java:224) [fabric-server-launch.jar:?]
at net.fabricmc.loader.launch.knot.Knot.init(Knot.java:141) [fabric-server-launch.jar:?]
at net.fabricmc.loader.launch.knot.KnotServer.main(KnotServer.java:27) [fabric-server-launch.jar:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_275]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_275]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_275]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_275]
at net.fabricmc.loader.launch.server.FabricServerLauncher.launch(FabricServerLauncher.java:62) [fabric-server-launch.jar:?]
at net.fabricmc.loader.launch.server.FabricServerLauncher.setup(FabricServerLauncher.java:106) [fabric-server-launch.jar:?]
at net.fabricmc.loader.launch.server.FabricServerLauncher.main(FabricServerLauncher.java:50) [fabric-server-launch.jar:?]
Caused by: java.lang.NoClassDefFoundError: net/minecraft/class_1044
at dev.tr7zw.firstperson.fabric.FirstPersonModelMod.onInitialize(FirstPersonModelMod.java:35) ~[FirstPersonMod-2.0.0-SNAPSHOT.jar:?]
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke0(EntrypointUtils.java:50) ~[fabric-server-launch.jar:?]
... 17 more
Caused by: java.lang.ClassNotFoundException: net.minecraft.class_1044
at java.net.URLClassLoader.findClass(URLClassLoader.java:382) ~[?:1.8.0_275]
at java.lang.ClassLoader.loadClass(ClassLoader.java:418) ~[?:1.8.0_275]
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:352) ~[?:1.8.0_275]
at java.lang.ClassLoader.loadClass(ClassLoader.java:351) ~[?:1.8.0_275]
at net.fabricmc.loader.launch.server.InjectingURLClassLoader.loadClass(InjectingURLClassLoader.java:56) ~[fabric-server-launch.jar:?]
at java.lang.ClassLoader.loadClass(ClassLoader.java:351) ~[?:1.8.0_275]
at net.fabricmc.loader.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:168) ~[fabric-server-launch.jar:?]
at java.lang.ClassLoader.loadClass(ClassLoader.java:351) ~[?:1.8.0_275]
at dev.tr7zw.firstperson.fabric.FirstPersonModelMod.onInitialize(FirstPersonModelMod.java:35) ~[FirstPersonMod-2.0.0-SNAPSHOT.jar:?]
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke0(EntrypointUtils.java:50) ~[fabric-server-launch.jar:?]
... 17 more

I know it's clientside-only but there should be a check like in other mods to disable it or do nothing when loaded on server to prevent crashes.

I'll have to check what needs to be setup in the fabric config.

Server? The first-person mod is clientside only. You don't need to (can't) install it on a server.