unlock with Apple watch

My iMac is unlocking with my Apple watch, but I cannot get my MacBook Air to do so. I have been through all the steps twice. I don't even have the choice in System Preferences>Security to choose "unlock with Apple watch." Why? What is going on? It is a work computer, but I am an admin as well as their admin, so doesn't seem like that should matter (I've added new OS's and apps with no problems). Anyone know why it isn't even a choice?