You missed some important details.
Exactly what RAM, what configuration? I'm going to assume DDR4, but you never know, people have done sillier things. But with AMD, the exact memory may be a factor, depending on the CPU.
Which CPU, not something you should leave out when expecting PC help. The motherboard will have entirely different capabilities depending on what is plugged into it.
So far we have A320m-k, and the RX550 Aero from MSI.
The rest of the hardware is just as important. Power supply, chassis, drives, etc. I can't even begin to guess what you have done wrong if you don't list out every last detail you have.
You could also try describing your build process. And try to think of everything you did.
Common mistakes:
Forgetting to install the CPU power cable
Installing a motherboard into a chassis without standoffs (when standoffs are required, some chassis have them integrated)
Not fully seating the memory
Forgetting PCIe power cables for GPUs (not in this case)
Installing an 8-pin PCIe connector into the 4+4 EPS CPU power connector
Plugging the display output to the motherboard with a CPU that doesn't offer video capabilities.
Installing the CPU in the wrong way
Installing an incompatible CPU (Very possible in this case)