according to solidworks help, I may use the "simplify" function ( 2018 SOLIDWORKS Help - Simplifying Parts and Assemblies  ) on assembly. There is nowhere written that the assembly should comply with some restrictions...

but in my Solidworks  "Tools > Find/Modify > Simplify" is greyed out, so I can't use the function.

I try to select a part, a set of parts, nothing... it is always greyed out.

I tried to fix all part (documentation speaks about a loss of contraints when simplified)

I tried to make alle part not "lighweight" apperance....

Nothing works.

The only "special" thing on my assembly, it is that comes from a STEP import.

By the way, I'm not sure I want to "simplify". I just want to have a single file "not 1000 parts and subassembly", less details, and a lighter file at the end)