The open assignment action should be passed the pzInsKey of the Assign-Worklist object, and not of the Work- case itself. If you have modified the OOTB worklist report, make sure the key that you are passing to the Open Assignment looks something like "ASSIGN-WORKLIST DMORG-SAMPLE-WORK S-123!CREATE". Anything else will not work.
I have a parent case and child case, I am only loading child cases in "D_pyUserWorkList". But, while creating child cases (From Web) I am fetching parent assignment key using (pyWorkCover.pxFlow(pyWorkCover.pxStageHistory(<LAST>) .pxProcesses(<LAST>).pxProcessName).pxAssignmentKey). and copying it to a property of child case.
I am using that property reference to open parent case from child case. this is not working and I am getting error "cant perform this operation when offline".
That error message is fairly generic and has many causes.
Based on your description, I am guessing that your D_pyUserWorklist only has pzInsKey entries for the child case. This means that only the child case work objects and assignments are packaged and sent to the client. So, when you try to open the work objects offline, and they are unavailable, the client is then trying to retrieve them from the server, which is unavailable, so you get the error you reported.
Assuming that your parent case is offline-enabled, you should list the parent cases assignments as entries in the D_pyUserWorklist, then if you do not wish for them to be listed in the worklist on the client screen, then you can hide them with a visible expression.
I am not able to include parent case assignments to D_pyUserWorklist client store, as the App is packaging only current user work pool cases. this scenario my parent cases are stored in different work pool and child cases in other work pool.