Only 1 device not connecting to R6700AX Nighthawk wifi router

We will need the model of the WiFi adapter of the PC that is having trouble connecting.
Beyond that, also check which channels the devices that the computer can't see  are using. For example, if your smartphone can see all of the APs, then run an app like WiFi analyzer https://play.google.com/store/apps/details?id=com.farproc.wifi.analyzer

and then see if they have any channels in common. Often when a single device can't find an AP, it is typically due to something like it being an older WiFi radio that never went through the approval process to use DFS channels, thus APs using those channels, end up not being visible.