I've tried all kinds of (16) Hex Keys SAE: 1/16", 5/64", 3/32", 1/8", 5/32", 3/16", 7/32", 1/4" Metric: 1.5, 2, 2.5, 3, 4, 5, 5.5, 6MM
but nothing fits these round screws, please help!
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The picture conceals all with soft-focus, but you don't mention trying Torx drivers, so that's probably what it is if you tried hex drivers but none of them fit.
From WonkeeDonkeeTools.co.uk, (no affiliation, but I love the name and the images from Wikipedia refused to load for me) the difference between Allen/Hex (top) and Torx (bottom)
Unless, of course, it's a 4-sided recess (yes the focus is that bad,) which is Robertson / Square Drive.
Or it has a pin in the middle preventing the tool from entering, in which case it's "security hex/torx/etc" that needs a driver with a corresponding hole.
If it's completely rounded out, other answers cover that case.
Unfortunately the camera focused on the wall tile and the ceiling heat lamp (ie well beyond the valve) so the image is quite blurry in the area of the screw.
Have a very close look at the screw. Is there any debris in its head? Mineral deposits perhaps? You might have some luck in cleaning it with a mild acid like vinegar. It might be possible to soak that spot by inserting a moistened cotton swab, leave it a while, re-wet the swab, leave it some more..
It might also be possible to clean the socket of the screw with a sharp tool such as a dental pick.
If you can be sure about the positioning you can try driving a hex key into the screw by tapping the key with a hammer.
If all else fails: yeah, extractor. But try drilling the hole for the extractor with a left-hand drill bit. The truth about me and extractors is that I've never had one work. I always twist too hard and shatter them. But a few times when I've had an urge to use an extractor it turned out that drilling with a left-hand bit did the job and saved me the frustration of dealing with yet another broken extractor!
Hard to tell since the pic is out of focus but you might be able to get a pair of small-nosed needle nose pliers in there to turn it counter-clockwise. If they won't fit around the outside try putting one prong on the outside of the screw and the other on the inside.
If that doesn't work you might need to buy a set of screw extractors like the set below. If you do, you may need to drill a small pilot hole in the center of the screw being careful not to damage the threads and use the smallest sized extractor. They're meant specifically for this type of problem and I've used them successfully a number of times.
Once it's out you can purchase a replacement hex screw.