I retract my previous comments about using selections sets of faces and the measure tool to get the surface area of said faces. Apparently that doesnt work either, which is very aggravating. I've got 1200+ faces I need the to find the surface area for and if I have to continue to select all these faces (or do an invert selection of the faces not needed 200+) anytime a part or assembly changes, I'm going to lose my mind!
I've gone through multiple times to make sure there were no stray edges, points, bodies, etc selected causing the "invalid combination". I've even re-created the selection sets using a "face filter" to ensure no stray edges were selected and still have no success. I guess we need an enhancement request for this one!