First change Gamertag, free ?

Hello Geoffroy schmitt,

You are allowed one free gamertag change as long as you didn't choose it yourself. So if you've been using the default gamertag when you setup your account, typically this will be a randomly generated name with numbers, your good to go, otherwise you will
need to pay to have your gamertag changed.

From PC:

1. Go to the Change gamertag page at account.xbox.com.
2. Sign in using the Microsoft account email address and password associated with your gamertag.
3. Enter your new gamertag, and then select Check Availability.
4. Once you find an available gamertag, click Claim it.

From xbox:

1. Press the Xbox button to open the guide.
2. Select your gamerpic in the upper-left corner of the screen, and then press the A button on your controller
3. Select My profile, and then Customize profile. Select your gamertag to edit it.
4. You can either select one of the suggested available gamertags, or select Make my own to create your own
5. When you're finished, select Claim it, and then select Sounds good.
6. Once you see your gamertag, select Close.

Also keep in mind you have 30 days to use your free change. It's not free after a certain amount of time has passed.

If you never change your Gamertag you have 1 free change, but only if your actual Gamertag was the Gamertag system assigned to you

It is only free if it is system assigned gamer tag. If you chose the tag yourself then you have used your free change.

Hey it sounds like you may have changed your gamertag once before and you may not remember when that occurred. You can contact Xbox Support to further assist you on or if you may have changed your gamertag previously. You may have a free change of gamertag
and it's not allowing you to access it. Sorry for the issue that you have experienced. Hope this was helpful info. Enjoy your day/night.