I’ve seen this problem elsewhere, I have ensured the file paths are the right ones, installed the plugin, took every step needed with* unreal engine open while exporting. No luck whatsoever, on either PC, this one has an RTX30 series so I know it’s not my specs. I saw a video where the plugin made a Megascans green button appear then his exports began working…but I don’t have said button. Any help solving this would be greatly and deeply appreciated/cherished. Thank you all in advance.
What does the log say? 
Wouldn’t have mattered bub. I fixed this with a workaround I found after much digging…so I’ll post it here incase others encounter the same problem(It makes no logical sense…but works) Thanks for asking though…even though I’m brand new to Quixel and couldn’t find the log for the life of me so I dug around instead. <3
I found our answer. Put the Plugin destination in your Project folder in a custom made Plugins folder(In the same folder as your .uproject) then reload Unreal…it will fail again but fret not for it will ask you to enable the missing plugins…that you have likely been DYING to see instead of it not working at all. Restart UE4, export, voila.
I pray this helps…I had to drive myself mad to find this out. This seriously needs added to the Documentation, don’t leave it as a guessing game. Much love and best wishes to you all!