This error typically occurs because your current Microsoft 365 subscription (like Personal, Family, or Business Standard) does not support "Shared Computer Activation". This feature is usually reserved for higher-tier Enterprise plans (E3, E5) or Business Premium.

If you are seeing this on a private/personal computer, it often means a setting was accidentally enabled that tells Office to act like it's on a shared server.

Solution 1: Disable Shared Computer Mode (Most Common Fix)

If you are using a personal license, you need to tell Windows to stop trying to use shared activation.

1. Press Windows Key + R, type regedit, and press Enter.

2. Navigate to the following folder:

   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\ClickToRun\Configuration

3. Look for a value named SharedComputerLicensing.

4. Right-click it and select Modify. Set the value to 0 (or simply delete the entry).

5. Restart your computer and try signing into Office again.