On the CT600 you need to click file / attachments and make sure there are no attachments listed, if so click remove.

On page 2 of he CT600 form you need to uncross the boxes at questions 80 and 85. Box 90 needs to be completed with a valid reason as to why you are not sending accounts and / or computation.

If you click the drop down menu it will provide a list provided by HMRC. You have a seperate drop down menu for Accounts and Computations, both need to be completed if neither attachment is being sent.