NOTE: Click on a picture to see it properly, sorry for the inconvenience 

Hello!
I'm here because I have a problem creating the wall boundary for my rocket design.

Whenever I try to pick the faces of my fins (called triangles in this assembly) I get the message:

"Face@Triangle-Y is not laying on the boundary between solid and fluid region."

I even tried to hide the body cylinder it is mated to and choose the hidden face, still get that message.

I tried running check geometry on the whole rocket, and on the fin only, and both never came back with result of 0.

Also made sure the fins are in the computational zone:

And lastly, the mates that connect the fin (and then I circular patterned it) are only between 2 faces (and not point or edges, which I figured might cause problems when it comes to CFD) :