Perhaps try and get the machine to boot with a single RAM stick in the correct slot using a stick that is known to have worked. Then at least you can get to the BIOS/UEFI and can check the info in it and try some alterations. Then you could progressively insert the sticks to see what fails.
These queries come to mind:
Does the BIOS/UEFI have reading of the speed of the RAM sticks, and what is it? E.g. 3600MHz, or some other value. In other words, is the BIOS/UEFI reading of the sticks congruent with the actual claimed speed of the sticks?
Is the speed of all the RAM sticks the same? They may have the same capacity, but what about the speed? Ideally, if they are the same speed, they co-operate seamlessly, but if they have different speeds, they will work only to the speed of the slowest, or in some cases not work.
Sometimes in the BIOS/UEFI, the DOCP (usually in AMD) is called XMP (usually in Intel), so XMP is likely with the MSI PRO B660M-A motherboard since it's running Intel (12th Gen), which uses the XMP initialism. So check post #4 of GatorsFan, substitute XMP for DOCP and perhaps try that.
Does a visual inspection of each physical memory stick show any differences? Are all the tiny terminals intact? Can you clean out the socket with some electronic cleaner (e.g. CRC Contact Cleaner pressure can)?
