In Singleplayer it works fine but if i try to join on my server it says:

incomplete set of tags received from server minecraft.Please contact the Server Operator.

This issue has been marked as stale because it has been inactive for 3 weeks. It will be closed if it remains inactive for another 3 weeks.

This issue has been closed since it has been inactive for 3 weeks since it was marked as stale.

Make sure that all the mods that add "Tags and Blocks etc" are added to the server.
Someone else said this about the same issue:
Make sure you launch your server with forge-xx.jar and not minecraft_server_xx.jar!

I am having the same problem - mod lists are identical for server and client, server is booted from bat file using following line java -Xmx10280M -Xms10280M -jar forge-1.16.5-36.1.4.jar

not a Create issue, if your mods are exactly the same on client and server check to make sure the forge version is too, that is the other most common cause.

Forge versions are identical as well - will see if I can isolate the issue more