Access Error in SharePoint: How to Troubleshoot SharePoint

Below are several of common errors in SharePoint Online and One Drive 

• "Error: Access Denied"
• "You need permission to access this site"
• "User not found in the directory"
• "Sorry, this site hasn't been shared with you."

What are the causes of the problem?

There are many incidents that might prompt one of these messages. The most cause might be because the permission for the user or administrator is configured incorrectly or not configured at all.

Solution of the problem?

Run the check user access diagnostic.

This feature required a Microsoft 365 administrator account. 

Click the link given Run a Tests: Check SharePoint User Access to diagnose the Microsoft 365 Admin Center.