I have a MacBook Air Wi-Fi turned off and will not turn on

Try restarting the Mac:

Often returns things to order.

(NB: may require your Mac password)

Eg: Click the black apple on the left in the top menu bar,

and

choose "Restart" from the drop-down menu that opens.

Wait for Mac to restart

Log back in

Check if you can now turn Wi-fi on ok.

Hi,

Following article could help your issue.

If you can’t connect to a Wi-Fi network on Mac - Apple Support