You haven’t connected in the last 30 days. Start the

This can happen if you take a long break from the game where the authentication cookie on your system for the game expires and you get stuck in a sort of weird authentication state.

The easiest solution is to delete that cookie and then relaunch the game to authenticate again.

Open File Explorer (Right Click Windows Start)
Type the following into the Directory Bar (not the search bar)

%localappdata%

Navigate from there to Blizzard Entertainment\ClientSdk

Delete the Cookie.bin file.

Restart the Desktop Application and game client to connect and re-authenticate.