Have a look at your postage preferences in your account and see if there is anything amiss:
https://www.ebay.co.uk/ship/prf
If that doesn't work, then when you edit the listing, refresh the page whilst on edit and that should clear the postage block issue. However, make sure that the P&P doesn't exceed the amount for the type of postal service provided, as the system won't save the postage amount you are trying to set.
There is a cap at around £11 for Special Delivery Services, so make sure you add any extra needed to the starting or BIN price.
Truth hurts. Maybe not as much as jumping on a bicycle with a seat missing, but it hurts.
You may think you have pushed ALL my buttons, but you still haven't found the mute!