Hey I have the same problem here . On my Laptop I can use Nanite and on my main machine I cant.
Both have the same graphics driver : 511.79 and the same windows version Windows 10 Pro 21H2 build 19044.1645
On both machines I have Direct x12 as the default RHI and the sm6 experimental ticked
The Laptop has a 3080 and the main machine a 3090 thats the main difference . I cant figure out why one is working and the other one isnt 