openmw.org

Hi there I've tried to connect to my friend's server but it keeps saying that I'm missing data files.

https://imgur.com/a/yhg5Ahs

As you can see they are not missing in the launcher, but every time I try to connect it only recognizes morrowind.esm, tribunal.esm and bloodmoon.esm. How do I fix this?

Check your relevant tes3mp-client log, and see what it says was missing/incorrect. The logs should be in your config folder.
You could also paste it in a spoiler here, or on pastebin, and I'll take a look.