Hi albehany, Merge can only merge edges that are tangent to each other. Those edges probably are not quite smooth and so can't be merged. If you could post the file I could check if that is why.

I'd probably try doing an "untrim" on that surface. To do that use Edit > Separate to break it into an individual surface not joined to anything else then select all edges and use Delete. That will remove the current trim boundaries and recover the underlying surface with its natural boundary edges.

- Michael