Hi Coalsand,
Thank you for reaching Microsoft Community.
Please try first to turn on Bluetooth on both your Desktop PC and the Surface Pro 6 then try projecting again if the the Surface Pro 6 can now be seen by the Desktop.
If still the issue persist, is there a third party antivirus installed on the Surface pro 6? If yes, kindly try to disable it first then try projecting again.
Is the Surface plugged in with the power supply when you tried to connect? If not, (on the Surface) please go to Windows Settings > System > Project to this PC. Turn off the This PC can be discovered only when it's plugged in option.
Hope this helps.