can't put assy to flex

hi,

In attachment is a assy of a hydraulic cylinder I made.

Afterwards I want to put a design table behind it for the different models and lengths.

For some reason I can't put this assy to "flex".

Always "sub-assemblies with layout sketches must be kept ridgid"

I did not make any layout sketches! Al the parts are made separately , then assembled.

main assy is N0058-003-000

thanx for any suggestions

nico

SolidworksAssemblies