I don't use deferred but looking at one of my scripts, the key points were in the following order;

1. SelectBatch (if batch if batching is enabled)
2. SetPostingDate (i just pass it the session object's SystemDate property which is already in YYYYMMDD format)
3. InitReportEngine
4. ProcessReport
5. UpdateInit
6. UpdateMain

Different update objects sometime vary and that might make UpdateCleanup needed. 

The one thing you'll likely need to do is output the retVal and objRegister.sLastErrorMsg to either a message box or a log file so you can see if an error is being returned.  You should also set the sLastErrorMsg property to a blank string before you call each method that you need to trap so you aren't seeing an error that was set my an earlier method.  You could also use If statements on the retVal values so you only proceed to the next point if the previous point succeeded instead of trudging through them all without regard for possible errors.

Also, make sure you covered the steps in this post if on an applicable version.

https://www.sagecity.com/support_communities/sage100_erp/f/sage-100-business-object-interface/150252/sales-order-printing-via-boi-in-sage-100/393580#393580 

Yeah I have a select batch in the code, just forgot to paste it there. also doing the same order as you indicated but in the ProcessReport the return is Nothing. Even I changed it to Print / Export

Also, we already done the pvxwin32.exe.config but the issue is still the same.

Try commenting out the following lines and also check the returned value of InitReportEngine along with the objRegister.sLastErrorMsg property.

oSSRegister.nTerminateUI()

objRegister.nPopulateWorkTable()

That work! I don't understand why disabling the two lines above solves the issue but thank you very much! Been working with it for more than a week.

It was likely the nPopulateWorkTable method causing your issue which means you can probably uncomment the nTerminateUI method.