Gamertag Change Request Failed

A failed friend request tells you very little. The only way to tell for sure if the gamertag you want is taken is to try to change yours to it, as you found out. It could be a difference in spacing: you are
Biscuit66, so I can't have Biscuit 66 even though a friend request to the latter would fail.

forced name changes only occur if the enforcement team deems your tag offensive. As yours does not you will need to pay the 800 MS points like everyone else to change it. If you are being harassed, I suggest reading the following thread to assist.

forums.xbox.com/.../19139.aspx

You will not get a free gamertag change.

Block comms & report anyone sending abusive messages via your console.

If you have already set comms to friends only then you wouldn't receive any more messages from people not on your friends list.

Hello again,

If you read the second link I posted it explains about forced gamertag changes in the suspensions and console bans section.

Gamertags cannot be requested. Even if the tag looks as if it hasn't been used there are other MS services that could be using it.