inavlid username

Yes and no. That is the reason why it's showing as taken, but there has been an update to the gamertag system so you can take the tag anyway, however, since someone already has the tag, it will add some numbers to it sort of like Discord usernames. Right
now the numbers will show until the update has been fully released. https://support.xbox.com/my-account/manage-gamertag-and-profile/gamertag-updates-faq

oh ok, thanks, guess my ocd will be torturing me for the days to come

If I recall, it basically shows that so all your friends know who it is that they’re talking to. Let’s just say for the sake of argument, you’ve got 200 friends. If you change your screen name/gamer tag, unless you go out and paying every single one of
them, it wouldn’t let them know that you changed it.

Unfortunately you'll have to pay to pick another
gamertag. Be careful when confirming your choice.

Hello, I am sorry that this has happened to you.

Unfortunately, if you have used your free username change, you will have to pay to change it again. According to support.xbox.com :

"If you're changing the gamertag we created for you at signup (meaning you didn’t choose it yourself), you won’t be charged. For all other gamertag changes, you’ll be charged (fee varies by region and currency)"

I apologize for the inconvenience, but I hope this helps.