Hi Jim - the surface edges are slightly out of tolerance (CrvDeviation is a good tester) MatchSrf for Position, with Refine set to Yes & ‘Match by closest points’ should bring it in close enough to join.
If the first surface (U-shaped) is realized with a 2-track sweep and then the flat surface with “Planar Curve Surface” using the curve the joint will fail.
But if you use the U-shaped edges plus the purple curve the joint will be possible.
if you use network surface it can quickly happen that surfaces dont join depending on how tight you set the tolerance. try if lowering the tolerance helps in creating a joinable edge. if you duplicate the inner boarder of the outer surface and make PlanarSrf it joins.