Sounds like the Bluetooth is not connected between your phone and S Pen. Plug your S Pen into your phone, and then do the following:
Settings | Advanced features | S Pen | Air Actions | Three Dot menu (upper Right) | Reset S Pen
This will take a little bit, but once done, your air actions should be restored. Note that this will not work if your S Pen is not inserted into your phone.
There are some additional steps to try here: https://www.samsung.com/us/support/troubleshooting/TSG01001830/
If you are still unable to access Air Actions, take the phone to an authorized service location for an evaluation. https://www.samsung.com/us/support/service/
Outside the U.S., click here: https://www.samsung.com/us/common/visitlocationsite.html