It seems that 2Peak isn’t an officially supported third party connection site for Zwift. I do see that 2Peak has a Facebook post explaining how to do the export.
While TrainingPeaks is a different site, I do know that purchased structured workouts from there don’t work for Zwift, so if it’s a structured workout from 2Peak, that may be why you’re having the issue.
Are the workout files .zwo files? .zwo files are usually the type of files used to add to Zwift.