But I thought that RAM compatibility doesn't matter too much so was perplexed at that myself.
It does matter but is still pretty flexible. There are too many RAM makers and models for motherboard makers to test and list them all so you don't have to buy listed RAM to ensure compatibility. But you should buy RAM with the same specs as listed RAM if you want to ensure compatibility.
I know Ryzen CPUs like slots 2 and 4 but no combination is working.
No. This is not a CPU function, but a motherboard function.
Slots can be physically damaged if the wrong RAM (DDR3 instead of DDR4) is "forced" in, or you attempt to "force" the RAM in backwards. But that does take some brute force.
Did you observe proper ESD precautions before touching the RAM? That is, did you unplug the computer from the wall AND
touch bare metal of the case interior to discharge any static in your body BEFORE
reaching in or handling the RAM? And did you only handle the RAM by the edges avoiding contact with the electrical contacts?
You did not say how your 8GB is configured (1 x 8GB or 2 x 4GB). If just one stick, according to your manual, it should be in slot A2. If two sticks, slots A2 and B2. See the image on page 7 of manual.
Did you triple check to ensure all power connections to the motherboard are there and tight? Your board appears to take the standard 24-pin and 1 x 8-pin 12V power connectors. Make sure both are connected.
Finally a common new builder (and distracted pro) mistake is to insert more motherboard standoffs in the case than the motherboard has mounting holes. Cases are designed to support 1000s of different motherboards. So there are almost always more standoff mounting holes than motherboard mounting holes. An extra standoff can short out the board.
If me, I would try another PSU just to make sure yours is working properly. If all checks out fine, then you may need to RMA the motherboard.