I un-sorned my car today at the Post Office – no forms to fill in, just hand over the V5C, MOT & insurance certificate and the money. Best to do it at the Post Office (you get your tax disc there and then). You are only allowed to drive a vehicle on the road without a tax disc displayed, if it was renewed on-line before the old disc expired – and even then, this exemption only lasts for 5 days. Therefore if you are un-sorning the car on-line, you will have to wait until the tax disc arrives and is displayed on the car, before taking it on road.
One possible reason for your difficulties is that the SORN form (V14) is based on you keeping the vehicle off road for at least the next 21 days (from the date of making the SORN). So if you have changed your mind within this time period, their systems might not be able to handle your request.