cannot add email alias to microsoft account

That usually happens for one of these supported reasons:

1. The email address is already associated with another Microsoft account. An alias cannot be added if it already belongs to another Microsoft account, even if it was created previously.
2. Existing @hotmail.com, @live.com, @outlook.com, and @msn.com addresses cannot be added as aliases to a different Microsoft account.
3. A work or school address may be blocked by the organization and cannot always be added as an alias.
4. If creating a new Outlook.com alias, there can be a delay before the new alias is usable.

What can be done:

1. Go to the Microsoft account Manage how you sign in page: Edit account info.
2. Select Add email.
3. Use one of these supported options:
   • Create a new Outlook.com email address and add it as an alias.
   • Add a non-Microsoft email address such as Gmail or Yahoo, if it is not already tied to another Microsoft account.
4. Complete verification.
5. If needed, select Make primary next to the new alias.

Important limits:

• An existing Outlook.com/Hotmail/Live/MSN address that was already created as its own Microsoft account cannot be attached as an alias to another Microsoft account.
• Aliases cannot be used to merge two Microsoft accounts.
• If the address was removed and it used a Microsoft domain such as @outlook.com, @hotmail.com, or @live.com, removing it permanently deletes that address and it cannot be reclaimed.

If the address was previously created as a separate Microsoft account, the fix is not to add that same address as an alias to another account. The supported path is to create a new Outlook.com alias on the target account or add a third-party email address that is not already in use by another Microsoft account.

References:

• How to add an email address to your Microsoft account
• Can I change the email address that is connected to my Clipchamp account?
• "Something went wrong" with error ADDriverStoreAccessNonLocalException
• Fixes or workarounds for recent issues on Outlook.com
• I can't add another new alias email to my account - Microsoft Q&A Why can't I add a alias email address which is available as like a new account to my personal outlook account as an alias.
• Adding a newly created outlook.com mail account to my Microsoft 365 subscription as main alias - Microsoft Q&A