Mirroring quilt and then merging overlapping error

Hi,

I am working on this surface that is part of an assembly. When I try to merge the quilts after the mirroring (in FRONT plane) i get an error of overlapping. I am struggling to find the overlap/how to resolve this issue. My files are linked (assembly, reinforcement plate and upperhood). The reinforcement plate is using the same geometry as the upperhood. The issue occurs in the part called: reinforcement plate. Any help is appreciated. 

I am using the student version of creo parametric.