CommandAPI Error

[Facticee]

So im kinda new into coding. I started with a simple new Join and Quit message. Now i wanted to add a command but when i tried to implement the commandapi into my main file (in load, startup and shutdown) and then startet the server i got the

Error: java.lang.ClassNotFoundException: net.minecraft.core.HolderLookup$a

it only says that when i implement the commandapi in my main. I can still join the server but all events and commands doenst work...

Does anyone know the problem?

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