The error relates to the tax office showing on the tax computation. This needs to be a valid 3 digit number.

You can enter it via clientbase by selecting the company, going into the business button and then entering the three digit tax office number. This will filter through into the accounts under client data / edit limited company data / CT. When you run the tax computation, at the top it should display the three digit tax office number followed by the UTR number.

If you are only using CT600s in Absolute, with the return open you need to click view / source. This will open a list of all entries alphabetically from the return. If you insert a row that reads district xxx (where xxx is the three digit tax office number) and click ok to save the changes. When you next rerun the tax computation you will have the tax office showing at the top of the screen, you will now be able to file the return online.