Valheim's incompatible version message usually appears when your game and the server you're trying to connect to aren't running the same version of the game. Usually, one side or the other has updated to a newer version of the game, while the other has yet to receive that update. The fix for this incompatibility is simple. Here's how to perform it.
Valheim Incompatible Version: How to Fix
The easier side of this fix is available when you're a guest joining someone else's server, but your game is out of date. Steam should automatically update your game when you're inactive, but it'll occasionally fail to keep up with those automatic updates despite its best intentions.
You can manually coax Valheim into updating by finding the game in your Steam library, right-clicking it, and selecting the "Properties" tab. From there, click on "Updates," and then click the prompt asking if you want to download and install updates.
Things are slightly more complicated if the server is what requires updating to a new version. To do that, start by installing SteamCMD. Once it's installed, navigate to the folder that contains your Valheim install and create an empty text file. Name it Update and change its file extension from .txt. to .bat.
Open that file and past the following:
C:\SteamCMD\steamcmd +login anonymous +force_install_dir C:\Valheim +app_update 896660 validate +exit
You'll need to modify that string if your SteamCMD or Valheim installation file paths are different. Once you've got that set up in your update.bat file, save it and execute it. This should update your server.