grey folder with ?

Hi I have a 2016 MacBook Pro ( non touchbar ) and recently I purchased a 1tb ssd for the MacBook in which I’ve had to purchase an adapter for it off ebay to make it work. The make of the ssd is WD Blue sn500 NVMe. Purchased from western digital. The adapter is a cheap one but recommended from a YouTube video I watched. Since re installing Mac OS numerous times I still have the grey folder flashing with the question mark inside. I’ve gone through Apple support they suggested brining it in for tech support as nothing they suggested worked. The grey folder is hit and miss sometimes it appears sometimes is doesn’t. When I leave the computer for a few minutes it goes sleep then it’ll go to the grey folder then I’ll have to turn off the computer by the power button and turn it back on. A thing I noticed is sometimes when I boot it on and it goes into the grey folder and I boot it back up and I press alt for the start up drive disk nothing will show which makes me believe it might be the adapter. This never happened with the Apple ssd it’s only this. could I get some insight of what it could be or how to stop this cheers