Welcome to the community! If you are trying to generate a custom report through your own survey project you will have the privilege to either work on the default report or create a new report (shown in the image below)
However, if you are trying to work on the custom report through a project shared by a colleague of yours (i.e. the survey project is not yours, and you have certain permissions as provided by the owner) you will be limited to work on the default report only (shown in the image below)
