Thank you for post!
I understand you are facing an issue with Azure subscription after signing up for a 30-day trial P1 license.
An Azure subscription can only be associated with a single Entra ID tenant, the other is a custom or additional directory that might have been created or added you can check the details and type of the tenant in the mange tenants tab which has shown in the below screen shot.
You have switched to your subscription to the new tenant, and you have noticed that membership type is grade out and you are unable to create the dynamic group.
To create a dynamic security group, you need to have an Azure AD Premium P1 or P2 license and also you need to have one of the following roles: Global Administrator, Privileged Role Administrator, User Administrator, or Intune Administrator for each unique user for more details check here.
The other side you have mentioned stating "got a message and why it is asking me to the Microsoft 365 admin center" the screen shot of the message and Microsoft 365 admin center page is not attached, could you please share the screen shots and steps you have performed so that we will help you on the same.
I hope this information helps! please Feel free to ask any questions you may have.
Reference: https://azure.microsoft.com/en-us/pricing/offers/ms-azr-0044p/
https://learn.microsoft.com/en-us/entra/identity/users/groups-create-rule
https://learn.microsoft.com/en-us/entra/identity/multi-tenant-organizations/overview
Thanks,
Akhilesh.
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