Panasonc G7 camera refused to take a photo

All AFs of today, CDAF, OSPDAF used by MILCs, or PDAF used by analog cameras, all need the contrast to decide in-focus or not. Some can do it with less amount of light (like your G7 and other Panys), some need more light (like the Sony, or nor pro class dslrs etc).

And the AF system might also be more sensitive to certain color, IIRC, red for example.

It is no different from the manual focusing back on the old non AF days. I remembered that if no straight line/edge in the intended focus point for the split prism to work on, I had to find something in approximate distance to do the focusing. The same apply today.

Yes, very often this piece of cloud might not be focused, but another piece can. It will just bet on our luck. As you expected, sometimes MF be the easiest for those situation. Unfortunately most native M43 lenses have no distance marking nowadays.

Regarding to use shutter priority than focus pirority, it might risk to result blurry image easier because when thevAF can't lock a focus, it will keep on moving in and out crazy (principle of CDAF) and so, the shot might actually fall on nothing...

Like other members suggested, sun set shootings are usually using infinity distance and so, lock focus on something you think is far away enough you will be good. I usually will use the mountain ridge, sea level surface, or even the reflection on the ground etc to lock the focus.