sounds, like you already got something wrong:
to use solvePnp() you need an existing 3d model (no, that’s NOT the result of it !)
IF you have coresponding 2d points, you can get a 3d pose from that
(think: a 3d face, and 2d face landmarks for the 2d points)
those are arbitrary in nature – it’ll lead nowhere (no corrspondance)