one of the slot not working

Hi,

Try to run Apple Diagnostics if it reports something related memory slot, though I just wonder if your memory suits spec. your MacBook Pro.

Use Apple Diagnostics to test your Mac - Apple Support

Did you purchase the memory upgrade from Crucial or OWC using the tools on their respective websites to get the exact part numbers compatible with your Mac? Macs are very picky about the memory they use so most contributors only recommend memory from Crucial or OWC.

You may have a bad stick of RAM or perhaps one of the modules is not fully seated or latched into place.

You don't mention the exact model of your Mac, but if it is a MacBook Pro, then the memory slot nearest the Bottom Case tends to fail due to bad solder joints that become cracked after years of pressure on the memory slot from the bottom case. I have seen a couple of cases where the memory slot nearest the keyboard goes bad too, but this is much less common.