Do you need to claim any reliefs at all in relation to the property disposal? If so which one(s)? Where and when exactly does the error show? Are you using the HMRC yellow worksheet?
Leave this with me while I go into my own tax account and see if I can re-create the error message?
Can you tell me what figures you have entered into the worksheet please?
When you completed the worksheet did you put a "0" into any of the other boxes? What does the worksheet give you as the gain or loss?
That's odd. Given that the worksheet does the arithmetic, its pretty much impossible to make an error as suggested by the message you got. You said you didn't wish to claim any reliefs so I guess you must have selected yes and then no. Otherwise, I would expect another error as the system will be expecting you to put a figure in the reliefs and elections box.
Apparently not. However, as there is a box for any relief you are claiming such as PPR, I would expect an error message if you don't put a figure in that box. Maybe a "0" or nothing in the box will be enough. If you change the answer to No, what happens? I realise you may wish not to tempt fate.
I have to say, that's got me stumped. If the tax works out that makes it even more strange. If you look at the CG pages here, you will see that box 35 asks if you are making a claim or election. That box will have PPR in it and if you read the notes for box 35 HMRC will be expecting you to explain that entry in box 38. I'd tell them that unless you made a claim, you got an error message.