If WM is active the Pick quantity field becomes grey out as this automatically gets populated upon confirmation of Transfer order, In the above thread you mention that you are able to change the pick quantity manually. Standard SAP rule is if you are planning to make the same Plant + Sloc combination to WM managed then there should not be any open document related to storage location.
The better approach in this case should be, Take help from development team to remove the indicator and delete the outbound delivery and again recreate and perform the full WM related activity.