@Catoer-Wibowo it can be that the activity has been rolled over if very old.
Open the activity on the watch logbook. Does it contain charts ? If yes it’s recoverable.forget the watch connection uninstall the app reboot the phone go to Bluetooth on the phone and make sure the watch is not paired. If not forget it again. Reboot the phone again if that is the case.
Install suunto app pair etc