Access to the SharePoint Store

Even though you're a Global Administrator and SharePoint Administrator, SharePoint Online has some additional permissions that might restrict access to the SharePoint Store.

Even though you're a Global Admin, Microsoft sometimes requires you to have the SharePoint App Catalog Administrator role to manage the SharePoint Store.

Your organization may have disabled access to the SharePoint Store at the tenant level.

The SharePoint Store requires an App Catalog site to be set up in your tenant. If it’s missing, you may not be able to access the store.

Sometimes, authentication issues cause "Access Denied" errors. Try:

Using a different browser (Chrome, Edge, Firefox)

Opening an incognito/private window and logging in again

If this works, you may need to clear your cache and cookies to fix it permanently.

If the issue persists, use PowerShell to verify your permissions.

My answers are voluntary and without guarantee!

Hope this will help you.