copperhopper-0.1.0+1.19.3
fabric-loader-0.14.13
fabric-api-0.69.1+1.19.3 - fabric-api-0.73.0+1.19.3
polymer-bundled-0.3.1+1.19.3 - polymer-bundled-0.3.7+1.19.3
polymer-enabled=true

Only copper-hopper, fabric api and polymer are installed on the server

[17:48:14] [main/ERROR]: Failed to start the minecraft server java.lang.RuntimeException: Could not execute entrypoint stage 'main' due to errors, provided by 'copperhopper'! at net.fabricmc.loader.impl.entrypoint.EntrypointUtils.lambda$invoke0$0(EntrypointUtils.java:51) ~[fabric-loader-0.14.13.jar:?] at net.fabricmc.loader.impl.util.ExceptionUtil.gatherExceptions(ExceptionUtil.java:33) ~[fabric-loader-0.14.13.jar:?] at net.fabricmc.loader.impl.entrypoint.EntrypointUtils.invoke0(EntrypointUtils.java:49) ~[fabric-loader-0.14.13.jar:?] at net.fabricmc.loader.impl.entrypoint.EntrypointUtils.invoke(EntrypointUtils.java:35) ~[fabric-loader-0.14.13.jar:?] at net.fabricmc.loader.impl.game.minecraft.Hooks.startServer(Hooks.java:62) ~[fabric-loader-0.14.13.jar:?] at net.minecraft.server.Main.main(Main.java:102) ~[server-intermediary.jar:?] at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:461) ~[fabric-loader-0.14.13.jar:?] at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:74) ~[fabric-loader-0.14.13.jar:?] at net.fabricmc.loader.impl.launch.knot.KnotServer.main(KnotServer.java:23) ~[fabric-loader-0.14.13.jar:?] at net.fabricmc.loader.impl.launch.server.FabricServerLauncher.main(FabricServerLauncher.java:69) ~[fabric-loader-0.14.13.jar:?] at net.fabricmc.installer.ServerLauncher.main(ServerLauncher.java:69) ~[fabric-server-mc.1.19.3-loader.0.14.13-launcher.0.11.1.jar:0.11.1] Caused by: java.lang.NoClassDefFoundError: eu/pb4/polymer/api/block/PolymerBlock at java.lang.ClassLoader.defineClass1(Native Method) ~[?:?] at java.lang.ClassLoader.defineClass(ClassLoader.java:1012) ~[?:?] at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:150) ~[?:?] at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.defineClassFwd(KnotClassLoader.java:153) ~[fabric-loader-0.14.13.jar:?] at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.tryLoadClass(KnotClassDelegate.java:355) ~[fabric-loader-0.14.13.jar:?] at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.loadClass(KnotClassDelegate.java:218) ~[fabric-loader-0.14.13.jar:?] at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:112) ~[fabric-loader-0.14.13.jar:?] at java.lang.ClassLoader.loadClass(ClassLoader.java:520) ~[?:?] at java.lang.Class.forName0(Native Method) ~[?:?] at java.lang.Class.forName(Class.java:375) ~[?:?] at net.pcal.copperhopper.CohoInitializer$ExactlyOnceInitializer.<clinit>(CohoInitializer.java:49) ~[copperhopper-0.1.0+1.19.3.jar:?] at net.pcal.copperhopper.CohoInitializer.onInitialize(CohoInitializer.java:30) ~[copperhopper-0.1.0+1.19.3.jar:?] at net.fabricmc.loader.impl.entrypoint.EntrypointUtils.invoke0(EntrypointUtils.java:47) ~[fabric-loader-0.14.13.jar:?] ... 8 more Caused by: java.lang.ClassNotFoundException: eu.pb4.polymer.api.block.PolymerBlock at jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641) ~[?:?] at java.lang.ClassLoader.loadClass(ClassLoader.java:520) ~[?:?] at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.loadClass(KnotClassDelegate.java:226) ~[fabric-loader-0.14.13.jar:?] at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:112) ~[fabric-loader-0.14.13.jar:?] at java.lang.ClassLoader.loadClass(ClassLoader.java:520) ~[?:?] at java.lang.ClassLoader.defineClass1(Native Method) ~[?:?] at java.lang.ClassLoader.defineClass(ClassLoader.java:1012) ~[?:?] at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:150) ~[?:?] at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.defineClassFwd(KnotClassLoader.java:153) ~[fabric-loader-0.14.13.jar:?] at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.tryLoadClass(KnotClassDelegate.java:355) ~[fabric-loader-0.14.13.jar:?] at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.loadClass(KnotClassDelegate.java:218) ~[fabric-loader-0.14.13.jar:?] at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:112) ~[fabric-loader-0.14.13.jar:?] at java.lang.ClassLoader.loadClass(ClassLoader.java:520) ~[?:?] at java.lang.Class.forName0(Native Method) ~[?:?] at java.lang.Class.forName(Class.java:375) ~[?:?] at net.pcal.copperhopper.CohoInitializer$ExactlyOnceInitializer.<clinit>(CohoInitializer.java:49) ~[copperhopper-0.1.0+1.19.3.jar:?] at net.pcal.copperhopper.CohoInitializer.onInitialize(CohoInitializer.java:30) ~[copperhopper-0.1.0+1.19.3.jar:?] at net.fabricmc.loader.impl.entrypoint.EntrypointUtils.invoke0(EntrypointUtils.java:47) ~[fabric-loader-0.14.13.jar:?] ... 8 more

I understand, thanks for the answer, when the developer updates the mod's dependencies and code, then it will be functional, because polymer 2 is on version 1.19.2

1.19.3 polymer changed a bit and this mod still targets old one. See https://polymer.pb4.eu/0.3.x/other/updating-0.2.x-to-0.3/

updated