Can I wear a Fenix 6s on upper arm (bicep)?

You’d probably need a large wrist band. There are some Velcro bands around that will still allow access to the WHR. Otherwise a simple Velcro strap and a chest him will work. 

You are much better off wearing a HR chest strap, maybe HRM pro or some other, start the activity, and then have your watch in an easily accessible spot. Belt loop, pocket, I do not know.

I have not found an HRM strap that has built in GPS tracking/storage without relying on phone connection.

I wonder if the HRM feature still works accurately on bicep.

There are OHR devices that do appear to work on the upper arm but whether the F6 would is something you’d have to try. I think you’d be better off getting a HRM strap if you want to be certain of getting usable heart rate data anyway, 

I think you misunderstood EliteJosh.  Garmin has chest straps such as the HRM-Pro or Tri that will track your HR and then download it after your activity.  You can start your climb, or whatever activity you want put the watch in your pocket, or backpack where it's protected and download your HR to the activity when completed.  It will still have your GPS track recorded.  Many do this for activities that don't allow a watch to be worn on your wrist.  Soccer, field hockey, etc... 

I have a Polar chest strap and an armband HRM. Both tracks heard rate nicely, but there is no GPS. I'm looking for one device that can track HRM and GPS in-unit without needing a phone.

The F6 worn on you upper arm with your polar chest strap or armband HRM will give you GPS and heart rate without the need for a phone.

The F6 alone using its OHR worn on you upper arm will track your GPS and might give you reasonable heart rate without a phone.

The F6 in a backpack and in range of your HRM-Pro or Polar strap or armband HRM will give you GPS and HR without need for a phone

The HRM-Pro will record your heart rate and make it available for download separately to your F6 if an activity is started on the watch but not in range of the strap afterwards but without GPS.

yes, it will work. i bought a set of strap that are like the ones for the enduro and one of them is a bicep strap. i tried it with my 6 pro and it worked great.

hi, would you please share link to your set of straps? i need a bicep strap!