That's brought up a memory for me.
I've had problems using certain numbers. In the C205, for example, there's the default, and there's 2 skins already in-game. I couldn't just choose C205_15, because it didn't like that.
I've noticed this in the 190s as well.
You can only go 1 number higher than the number of skins already in the game. If you have a plane with 2 skins already, you need to make yours "_3". If you have one with 2 skins already and already have 2 skins in progress using _3 and _4, you need to make your next one _5.
The P51s are full, though, all 15 skins are in there. I don't think it should be a problem with this plane.