see if you have a bluetooth on your desktop and it is turned on. go to settings and then go to bluetooth. if the bluetooth switch is off, toggle the switch to on.
also, move the desktop as close as you can to the TV on which the receiving Miracast dongle is installed so it is within the (short) bluetooth range. Sometimes for me the range is very short. a web search says 5-10 meters but i have experienced fails if it is spanning more than a few feet.
also try connecting the receiving Miracast dongle to your desktop to initialize it. that way it is super close. then after initializing it, switch it to the tv. (i have never done this and i have never messed with miracast so it might not work, or at least not work the way that i am guessing it works).
It is the laptop that had Bluetooth. The desktop never did (but I have wired audio connections to it, as the whole desktop setup is stationary.)
Bluetooth on the laptop has disappeared from settings and device manager.
The only reason the desktop came into play was that I was wanting to log into the router, as part of trying to get Miracast working on the laptop. (Miracast WOULD be cool to get going, as then I could “cast” not just audio, but video, from the laptop.)