If you can access the site as the incorrect guest, follow these steps: Step 1: Determine which account has access as a guest To fix this issue, determine which account accepted the invitation, remove the incorrect account if necessary, and then reinvite the user to the resource.
When a guest accepts a SharePoint Online invitation by using another account Then grant the user permissions back to the file or site. Make sure that the user is removed from the site collection. If there are still errors with the account, we recommend that you completely remove the guest account from the Microsoft 365 admin center. If the user doesn't have appropriate permissions, grant them permissions to the file or site. Verify the permissions by using the Check Permissions feature: If the user is trying to access another user's OneDrive site