I've set one Office 365 account to Global Administrator.
When that user tries to access SharePoint Admin page (domain-admin.sharepoint.com), he gets "Access Denied".
What could be the problem?
try the below steps
and now Global Admin user go to SharePoint Admin Center without a license.
If above did not resolve then you have to contact the Customer Support. Can't access Office 365 online apps
I had this issue. At the same time I had the issue I was testing adding a proxy address that was a duplicate of another user's email. I was hoping it would work since the user didn't have any type of MS license that included email. That user only had a Power BI per capacity license and a fabric license.
Once I removed the duplicate proxy address and synced our active directory to azure, I was able to access the sharepoint admin center.