On Shop floor workbench there is a tab called Tracked Structure used for serialized shop orders. You probably need to disconnect from there first.

@mmoss Thank you for your reply.

The tricky thing is, this shop order is not a tracked shop order, the parent part is not tracked, all components are not tracked parts as well. so there is no any tracked structure at all for this shop order. please refer to below screenshots.

In that case I am not sure what it could be.

was the original parts reserved and issued from the correct location?

I have seen a couple of reasons you can't unissue parts when;

1/ Original component was issued from stores and not the correct workshop/floor location (different authorization rules for different locations) the system tries to place the component back to the original issued location it came from and if you don't have authority to issue back to that location, it will error. even though you may have been able to issue the part.

2/ If a Part certificate has been produced. Or the item has been sent out on a custoker order etc. Once this function has been actioned, then it can be near impossible to unissue parts. 

This issue has been resolved by the Patch 167262 in APPS10 Track in the UPD22