ほかのユーザーがこのMicrosoftアカウントをつかってないのに「このデバイスの他のユーザーがこの Microsoft アカウントを使用しているため、ここに追加することはできません。」ってでてしまう

Hello, thank you for reaching out to Microsoft Q&A.

I understand that you are trying to switch from a local account to a Microsoft account. However, you are receiving an error "Another user on this device uses this Microsoft account, so you can't add it here."

This error appears if the Microsoft Account that you're using is already registered and active on the computer. To check that,

1. Click Windows + R key, type lusrmgr.msc, and click Ok.
2. In the left pane, double-click the users folder.
3. If you find a username similar to your Microsoft account, right-click the user and delete it.

Important - Before deleting the user account, make sure you take a backup of the data in the user account.

You can go to Local disk C:/ Users folder to take a backup of the user folder you are trying to delete.

Once you've deleted the user from the Local Users and Groups window, switch to your Microsoft account.

Let me know if you still need more help.

Windows のエディションが Home で lusrmgr.msc で「ローカルユーザーとグループ」が起動できない場合は、コマンドプロンプトで

　net user

を実行して、コンピューターに設定されているユーザーの一覧を確認できます。