how do I enable autosave activity

To my knowledge that is not possible (even on a 965) . You have to select one of the options on the screen after stopping,

The user manual says: " After stopping the activity, the watch saves it automatically after 30 minutes."

I don't think there's a way configure this time period though. It was actually an annoyance for me on a Venu 2 that didn't have resume later when it would save an activity while paused at e.g. a coffee shop.

Great. Thanks! I will just stop the run and let it autosave next time.
Here is the reference from the user manual in case others want to see more detail:
www8.garmin.com/.../GUID-7D2E0DD7-438F-4362-948F-000FCD0AF4D2-5144.html

My Garmin Forerunner 165 did not auto save a walk and the device missed saving my sleep that night, because it was waiting for me to save the activity. Is there any help to prevent this from happening again? I also notice that if I accidentally begin an activity it’s a process to delete and the device keeps on starting the same activity which needs to be deleted. Any help will be much appreciated 

3209990 said:

I also notice that if I accidentally begin an activity it’s a process to delete and the device keeps on starting the same activity which needs to be deleted.

Can you be more specific . If you start an activity in error, just  stop it and discard it. (dont save it, or leave it in stopped mode)

As to the first point just save the activity after stopping it , it is an easy enough thing to do straight away , rather than wait for it to save.

My forerunner 45 would auto save any activity after a certain amount of time, in case of forgetting. Is this no longer a feature?

right after I delete an activity started in error my device starts another of the same activity which then also needs deleting. Is this normal?