Since i’ve just seen the same post on reddit i’ll give you the same answer that was already given to you.
You’ve missed the app migration deadline and have to manually recreate your apps.
If yo do not see any apps from the truenas apps catalogue have you checked if you enabled the app trains under configuration → settings? And did you manually refresh the apps catalogue?
when go to apps the settings i see Help: Preferred Trains
Please specify trains from which UI should retrieve available applications for the catalog. it doesn`t appear like you