Hi Codeen
Thanks for reaching out to Microsoft Q&A Forum support.
Based on your description, I understand that you are experiencing a sign-in issue with Unmanaged device to the SharePoint Admin Center, even though you have administrative privileges. So, I think you can try.
Option A: Try to sign in with the managed device and unable blocking access in Entra ID
You can follow the instruction in this article: IT Admins - SharePoint and OneDrive unmanaged device access controls - SharePoint in Microsoft 365 | Microsoft Learn
Option B: You could use following PowerShell to allow full access
$AdminSiteURL="https://tenant-admin.sharepoint.com"
$Cred = Get-Credential Connect-SPOService -URL $AdminSiteURL -Credential
$Cred Set-SPOTenant -ConditionalAccessPolicy AllowFullAccess
Additionally, I've conducted some research and found this discussion is relevant to your context, you can find more insight here: Access denied on Sharepoint admin center but user has SP admin role | Microsoft Community Hub
All the method may need 24-48 hours for applied so if the issue persists after this period and after trying the above steps, please let us know.
Hope you will solve the problem soon
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