Thank you for posting your query on Microsoft Q&A, I could understand that you are trying to add an Azure AD subscription to your office 365 tenant, with technical notification being set to a personal account (gmail.com).
Please do let me know if this is not the case in comments section..
This is because a personal email address is not accepted as a technical notifications contact for an org.
As a work around you could change it to any work email address and once the subscription is added you could save it back to gmail account.
Thanks,
Akshay Kaushik
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