According to players' feedback, we have known that players can't log in and all WOW Classic realms show up as "Incompatible" owing to Classic client update. In this case, Blizzard suggests that you can restart the Blizzard app to solve this issue.
Incompatible WOW Classic realms appearing after a patch
It has been revealed that incompatible realms may be seen when you log in to World of Warcraft Classic after updating WOW Classic client. All realms turn red and appear as "Incompatible" because your game client is not suitable for the version on WOW Classic servers.
Restart Blizzard app to fix WOW Classic incompatible issue
Players have reflected that they can't log in and WOW Classic realms are also incompatible. In this case, Blizzard has come up with a solution to this issue that you should restart Blizzard app, and the detail steps are as following:
1. Open the WOW tab in the Blizzard Desktop App and choose "Options" and "Check for Updates" to check for Downloads manually.
2. Restart Blizzard App
3. Please ensure that Agent is closed.
4. Then close out the Blizzard Desktop Application
5. Next open your Task Manager with Ctrl+Shift+Esc
6. Look for Agent.exe and hit "End Process" if present. And you need to look under the "Details" tab if using Win10.
7. Reopen the Blizzard App and allow it to update
8. Run the Scan and Repair tool.
9. Open your Config.wtf file in the WTF folder and ensure your 'SET portal' line has "US" listed.
If these steps don't solve the problem, you can contact WOW Classic team online to fix this issue.
Have a try with those steps above. By the way, you can buy cheap WOW Classic gold from us with fast delivery.
The WOWClassicGP Team