Make sure the device you want to pair is actually a Bluetooth device. Ok look, we know this sounds like a silly suggestion but there are many wireless devices available that use proprietary wireless adapters in order to communicate with your computer and people often assume that these devices are using Bluetooth when they’re not.
To confirm; if your mouse or similar input device came with some kind of little adapter that needs to be plugged into the computer (usually into a USB port) then it’s highly unlikely that it supports Bluetooth.
A good example of this confusion is with mice. Actual Bluetooth mice will always have a pairing button on them to instigate the pairing function Bluetooth needs to go through to connect the mouse with your computer. If you have a wireless mouse that doesn’t have such a button or any reference to the Bluetooth standard (either written somewhere on it or a logo reference) then it’s very likely that your device is not actually using the Bluetooth standard. This check can be applied to almost any other wireless devices. If it doesn't have a pairing function or Bluetooth logos you can probably assume it's not Bluetooth.
If you're unsure double check that you didn’t receive a small USB adapter with the device that was meant to be plugged into your computer in order for it to function.