Why can’t I use some of the 3 character gamer tags that are available but not allowed

it is pretty obvious why that gamertag would be unavailable, besides most of the three letter names would be used anyway as there is only a finite amount.

https://www.urbandictionary.com/define.php?term=FFS

It comes down to whether they are acronyms for phrases that are against the Xbox Code Of Conduct.

If you're unable to use one then you may need to find a different GT to use.

Believe me there are such programs, i looked it up and there's numerous versions of them, these "Gamer Tag Turbo programs" that these people use to snatch the gamer tags that are 3 or 2 characters have many versions, some can find GT's quick and some just
scan to see which GT's are available, people can also make there own turbo sofware. Seeing as your an Xbox moderator you can look it up for yourself, because yes i know it's possible for someone to get a tag the moment someone changed there tag, just not milaseconds
after they change it, its just not possible because i had both my profiles open to make sure i change my name as fast as possible before anyone takes it.

I don't think changing your gamer tag to your personal name is a good idea.

However, Microsoft has been buggy since the gamer tag multiple change feature has been available.

Nothing to worry about Try again later or switch the gamer tag characters around.