Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Looks like you only changed the alias, not the security information prior to this. You should have gone to https://account.live.com > Security > Manage how I sign in to update there as well.
I don't think you can do that now as it wants the code sent to Gmail. If your password is correct, look for an option like “Show more verification methods” to see if you have a backup (e.g., a phone number or alternate email).
If you don’t have access to any of the options listed, select “I don’t have any of these.” This will begin the recovery process. As part of that process, Microsoft will let you add new contact information — but only after a 30-day waiting period.
This 30-day delay is intentional. It’s a security safeguard designed to protect users in case someone else tries to hijack their account by changing recovery info. The wait gives the rightful owner time to notice and stop unauthorized changes.
Unfortunately, this policy cannot be bypassed. Microsoft explains it further here.