There are a few reasons as to why you may be unable to reverse a delivery.
If the delivery has attached invoices:
You will need to cancel any invoices attached to the delivery before you can reverse it - see this article for more information on cancelling sale invoices, and this article about cancelling RCTIs.
The counterparty may have to reverse it:
For buyer deliveries, only the purchaser/receiving party can make the reversal. For further information see who can reverse deliveries.
In this case, you should contact the counterparty to reverse this delivery.
You may need to reverse the twin delivery (ie (-D):
If you can't reverse a delivery that's on an Order, it might be because you need to reverse its twin. This can be found using the Order Number to search Transactions > Deliveries. Once its twin has been reversed, your delivery will be automatically updated to 'In Progress'.
Check the Bin Segregation, if the Commodity/Grade have changed, this will block a ticket from being reversed (SO)
Relevant to Site Operators reversing Deliveries back to their Site bins, when details such as commodities and grades have changed after a Delivery has been out-turned, the 'Reverse' option is blocked.
Review your Bin Seg on the Delivery ticket in question, then check the Bin details in your Storage > Inventory to be sure this is the case, so appropriate changes can be made to rectify.