There is a innactive account

Hi RyderHunlin you can't use if the gamertag is already used by someone except since the last update where an suffix (like #001) come behind
your name.

Here the update explanation : https://support.xbox.com/en-US/my-account/manage-gamertag-and-profile/gamertag-updates-faq

Microsoft has said that it will delete inactive accounts that have been made dormant for a certain period of time, so it is just a case of being patient and seeing if the gamertag you want will become available in the near future. If you want this name
for your account then why not try putting numbers either before or after envy to see if that variation of the gamertag is available for you to use right away?

Hi RyderHunlin,

You certainly mean a gamertag and not an account. Just because someone isn't playing, the corresponding microsoft account isn't necessarily inactive. Maybe he's just sending emails or use skype or use a windows mobile phone. But the gamertag will not be
reassigned until the corresponding account is deleted (and then there is a time of contradiction).

Read more here
"How to close account"

Hope this helps and please keep us informed

here is the link to the new gamertag system as well. First change is free
https://social.xbox.com/changegamertag

Microsoft accounts is used on many products that Microsoft has, not just Xbox. Skype, Mixer, Microsoft Outlook, Microsoft Office etc.