It's a glitch in the servers since PS3 and PS4 and vita somewhat share the same servers.
So if you or the person you're trying to add are playing PS3 but one of you have a PS4, that will happen most of the time, it's not just you. It does that to me everytime I try and add someone because I have a PS4 also, which is quite weird. Anywho, it's not just you. More than likely everyone is going through this problem.