Hi,
This happens because some Microsoft accounts, including Teams Free, restrict certain domains from being primary, even if you can verify them with an OTP. Make sure your iCloud address is added as an alias at https://account.microsoft.comunder Your info > Manage how you sign in to Microsoft, and verify it if needed.
If it’s added but still won’t become primary, the system may be blocking it due to account type or domain restrictions.
Well, I suggest waiting a few hours and retrying.