...I am adjusting this setting ... on i-ttl green page of the Godox.
This doesn't sound like your V860 II is set up properly to be used as a Nikon CLS/AWL optical slave to the 800D's pop-up flash in commander mode. The V860 II has five different triggering modes, which you cycle through with the horizontal lighting bolt button on the right. They are:
- On-camera mode (no wireless): no icon, green backlight
- CLS optical commander: lightning bolt icon, green backlight
- CLS optical slave: lightning bolt icon, orange backlight
- radio master: antenna icon, green backlight
- radio slave: antenna icon, orange backlight
You need to set your flash into CLS optical slave.
The S1/S2 optical slave modes are "dumb" optical slave modes. It's like Nikon's SU-4 mode. It simply fires the flash whenever another flash burst is sensed, so will also work with your D800's pop-up flash. But there's no other communication and the D800's pop-up cannot communicate TTL at all. The S1 mode fires the V860 II on the first burst sensed; S2 on the second burst sensed so it can skip a TTL metering pre-bust. But with CLS/AWL, which uses multiple pre-bursts of light to communicate settings before sending the fire signal, the timing may also be off unless the flash is set to a power level low enough that it can recycle between the pre-flashes for the main burst.
One last note, be aware that pop-up flashes typically can't do HSS (they're too low-powered). So, they won't be able to communicate HSS as wireless commander/master to off-camera slaves, either.