Leaking ball valve at flush port
I agree with Jimmy Fix-it. You'll need to replace the valve entirely. It's a specialty assembly and not available to the public readily...that I've seen.

Can you just replace the seal on the grey cap and screw it back on to let it ride? IF so and everything's fine without the simply annoying drip, then take that road.

A Plumbing Supply store may be able to get you a new one. But, you need to figure out with them if you'll be sweating a new one on or going to a compression fitting, which needs to be longer if the old one's being cut off.

Alternatively. Is this part of leased equipment from the propane dealer? It may very well be a stretch. But, if so, then contact them and they will, typically, remedy the problem for free.

Of course, a Plumber might be your best bet in correcting more than this question. Is that valve assembly required or from another setup? Are those wacky reducers and not-yellow teflon tape okay? Is flex-line suddenly approved for outdoor use in a permanent installation? ...it's not, around me.