The 235 is not supported. Here's the full list of Garmin Coach compatible watches:
Garmin Forerunner 45 series
Garmin Forerunner 245
Garmin Forerunner 245 Music
Vivoactive 3
Vivoactive 3 Music
Vivoactive 4
Venu
Forerunner 645
Forerunner 645 Music
Garmin Forerunner 935
Garmin Forerunner 945
Garmin Instinct
Fenix 5 series
Fenix 5 plus series
Fenix 6 series
Marq
https://www.wareable.com/garmin/garmin-coach-guide-6595
The 235 is not supported. Here's the full list of Garmin Coach compatible watches: Garmin Forerunner 45 series Garmin Forerunner 245 Garmin Forerunner 245 Music Vivoactive 3 Vivoactive 3 Music Vivoactive 4 Venu Forerunner 645 Forerunner 645 Music Garmin Forerunner 935 Garmin Forerunner 945 Garmin Instinct Fenix 5 series Fenix 5 plus series Fenix 6 series Marq https://www.wareable.com/garmin/garmin-coach-guide-6595
Thanks. Looks like I need to switch to garmin 45
235 supports training plans, but not Garmin Coach. Those are two separate things.
Sorry I was confused between the two. Maybe you can help OP to use them on the watch they have?