Steam Store: you can't buy games for your friends?

I tried, it allows.

I think you can't gift cross-currency anymore. For example if your store is in Euro you can't gift to someone in Denmark despite both being in the European Union and your store prices being higher than the Danish ones. A potential lawsuit waiting to happen.
I am 100% sure that I can no longer gift to the US but used to be able to do so.

contact support it will be fix
i think this because reseller thing

Since August (as I remember) some people were banned from gifting because Steam saw some sort of "suspicious gifting activity" on their accounts, i.e. they were gifting a lot of games to a lot of different people.
So if you're not a seller of this kind, you can try contacting Steam support, they may remove this ban from your account.