Hello @Pontus Löfgren :) ,
Thank you for reaching out Microsoft Q&A.
To create a new tenant in Microsoft Entra ID, you need to have an Azure subscription. If you don't have one, you can create a free account before you begin.
Regarding your specific issue, it seems that the create button is disabled when you log in with your personal Gmail account, but it is enabled when you log in with your non-profit email address. This could be because the user account associated with your personal Gmail account does not have the necessary roles to create a new tenant.
To create a new tenant, you need to have the "Tenant Creator" role assigned to your user account. This role can be assigned by a Global Administrator in your organization. If you don't have a Global Administrator in your organization, you can create a new Microsoft Entra ID tenant using a different email address that has the necessary permissions.
Regarding your question about leaving the other tenants, it depends on whether you need to keep access to those tenants. If you no longer need access to those tenants, you can remove them from your account to avoid confusion. However, if you still need access to those tenants, you should keep them in your account.
Just for the confirmation can you share me the screen shot of the subscription page by hiding the sensitive information.
For additional Information: Create A New Tenant , Built in Roles
I hope this helps! Let me know if you have any further questions.
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Regards,
Goutam Pratti.