SharePoint - You need permission to access this site.

You need permission to access this site (when user already has permission)

Go to the SharePoint site | Site access | Advanced permissions settings.

Click on any of the SharePoint Groups and change the URL so it ends with MembershipGroupId=0

https://grangerconstruction.sharepoint.com/sites/HumanResources2/_layouts/15/people.aspx?MembershipGroupId=0

Remove the user who is having the issue.

Go back into the SharePoint site | Site access | Advanced permission settings and select the SharePoint Group the user should be a member of, add the user then wait a few minutes. You should now be able to access the site as the user.