Sounds like you’re trying to buy something from a region you’re not physically located in. This is fraud and obviously a violation of the Steam subscriber agreement.

If that isn’t the case, have you changed your store region? You should NOT be using a VPN to make any purchases

As above says ..
Using a VPN on Steam is a violation of their Terms of Service, and while Valve (the parent company of Steam) can ban or restrict your account for it, they are more likely to take action if you use it to purchase games from a different region or bypass regional pricing. Valve can block VPN IP addresses to prevent circumvention, frequent changes can trigger account locks.