If you’re referring to the wahoo app and not the systm app, then the export as fit file and export into garmin connect used to work until very recently, but about a month or so ago it stopped.
When using a iPhone the filter on the import screen in garmin connect failed to pick up fit files, and on a laptop, iPad etc the import would find the fit file but the import would fail. Total pain.
i use garmin and wahoo, but would prefer to keep all my exercise records in one place in garmin connect ( it’s not perfect, but it is improving)
one solution is to use trainer road or wahoo systm, but they are cost options, for something I don’t really need.