Can't get my mates to join my minecraft server

I suppose the problem is that you did not port forward the server. I know that it is necessary in order to allow your friends to join, but I myself have never done that, so I may not be able to help in port forwarding, but I can link a few tutorials to you which might help.

How to setup a java 1.16.5 server.

Port forwarding for any Minecraft version.

Also, to resolve your second issue, that is the message that says "server can't keep up, x ticks behind" means that you have not allocated enough ram to your server, due to which the server is experiencing lag. To fix this, the following links might help.

Website link (non video tutorial)

Video link (video tutorial)

If you already port forwarded, but still your friends can't join, consider reporting to Mojang about this issue. I hope this helped, if not, sorry.