I have some clamps that are part of a subassembly that I need to align with some legs within the main assembly.

The black parts in the picture are all part of one subassembly. I need to line up a heap of legs (grey) with the SHS you can see highlighted as blue using the width mate. Once they are lined up, I then need to lock the SHS in place so when I open the subassembly, it is in the same place (i.e. no difference between flexbile and rigid subassembly)

Though when I try to mate two parts of  the same subassembly together, while it is in the main assembly, I get this 'error.

Can anyone please help with this? I can't send the files unfortunately as it is very large.