can't verify existing xbox live account

Hi! Can you kindly add a little more information to the question?

Are you trying to add an existing Xbox Live account to your Xbox 360?

If you are attempting to add an existing Xbox Live account to an Xbox 360, Create or go to the profile you would like the existing Xbox Live account to be accessible on, go to Xbox Live and download the existing account to the profile.

Verify info to a gamertag that doesn't exist therefore there is no account at all?

It's not possible to merge existing live accounts together. So short answer is: You can't.

Xbox live does not have access to or is able to verify anything with a Youtube channel/account. They rely on the information you provided when you created the account. If you can't remember the account info - First and last name, address, phone #, answer
to secret question - then they have no way to access the account or verify account ownership.

You see the following error code and message when you try to sign in to Xbox Live:

Account does not exist.

Status Code: 8015D002

This may mean there’s a problem with your account. For example, the security information for your account may need to be updated.

Solution 2: Verify your Microsoft account security information

Follow these steps:

Visit Microsoft account.

Note If you forgot your password or can’t sign in using your password, reset your password.

Click Edit security info.

Verify your security information. For example, verify the Phone number, Alternate email address, and Security question information.

Sign out of your Microsoft account and then sign in again.

Try to sign in to Xbox Live again.