Ensure your host PC has Sunshine or NVIDIA GameStream enabled.
Often happens on legacy devices like Steam Link; double-check that your resolution matches exactly on both ends. Under the Moonlight v1.0 [Completed]
Download the Moonlight.nro file and place it in the sdcard:/switch/Moonlight-Switch directory. Ensure your host PC has Sunshine or NVIDIA
Switch to Desktop Mode, search for Moonlight in the Discover store, and install it. and install it.