I cannot pair the Magic Mouse to my mac

Try plugging the Magic Mouse into the Mac via the USB-to-Lightening charging cable. I was able to pair my Magic Trackpad to my old iMac using the Lightening cable. After it was seen by the iMac, I was able to use the Magic Trackpad without the Lightening cable.

Maybe you can get the same thing to happen for you with your Magic Mouse.

Did you unpair it from any previous device?