Currently, accidental torsion analysis is only supported when using the Standard response spectrum excluding Eurocode, which needs to be specified. The functionality is not available for Custom spectra, which is a critical limitation that needs to be addressed on priority.
Thank you for posting this idea, we have it in our backlog. Note that there is another similar request posted. It is often better to vote for ideas that relate to the same theme as those that have the most votes become more visible rather than having lots of similar ideas with only one or two votes.