This is a Piston, a Piston Pin and a Connecting Rod created in an assembly file which is going to be a sub-assembly in another assembly file.
Connecting Rod is moving when it was in an assembly file. It was not moving when it became a sub-assembly in another assembly file.
Is there any way to fix this problem?
Can you please tell me where about the flexible command?
RMB the sub-assembly.
Same as in ....
Do you mean Float/Fix command?
Of course you must Float (un-fix or (think un-ground)) the sub-assembly if you want the piston to also move within your block.
I assume in your actual assembly (of the engine) your sub-assembly was already Float, but in the example you attached here it was Fixed.
But to get the Connecting Rod to move in main assembly you must set the sub-assembly as Flexible.
Note that I did not say to change anything within the sub-assembly itself. You have that constrained correctly.
These instructions are only for action on the sub-assembly within the main assembly.
Float (not fixed) and....
Set the sub-assembly as Flexible.
same as in....