I've been trying to join a friend of mine for days, but every time I only get the message client out of date. My minecraft is also up to date.
Your friend is probably not playing the latest version. Create a profile with a different version and / or ask your friend which one he is using.
The message is a bit misleading as it affects old versions too, so you play the latest, but your friend doesn't.
If you play on Bedrock / PE it actually means that your version is out of date. Maybe try reinstalling Minecraft. It always worked for me in the past. But I have no idea about Java, because it was never there for me, and you can change the version there.
