I recently get a Samsung Galaxy Tab Active2 with Android 9 system.
I install When Last (v2.21) app from Google Play Store, delete all visible (default) items then set to sync with Dropbox, authorize with Dropbox (app version 170.2.8, installed from Google Play Store) with Dropbox Plus (paid) account. But it does not sync yet.
Status report: Waiting for initial Dropbox sync to complete. I know it takes a while, but it takes already 3 full days.
It's been years since last time I set up When Last sync with Dropbox (on another ancient Android tablet). I don't remember how long it shall need to initial Dropbox sync.