Never worked in the 3 months I have had the watch despite that option being enabled.
Toggle is active on the watch but , for me , advanced option is greyed out in Health app on phone.
Samsung has acknowledge it is an issue.
Response form Samsung
Posted about a week ago....
"Hi all.
The developers have advised us that they've identified the issue and added exception routine code for Inactive Time in the next software update of the Galaxy Watch.
When that update comes your way, can you let us know how it is for you?"
Normal service has been resumed thanks to update installed yesterday evening.
Bear in mind you can configure the day and a window for time of day the inactive reminder is enabled.
At 08:00 this morning I thought t still wasn't working. Default is 09:00 - 17:00 Mon - Fri
I also set Inactive time alerts to " Urgent:sound and pop up on screen" other wise you may just get a notification that you have to access the pull down menu to see.