It works now thank you both.
The problem was that I had understood that the livery needed to be in this directory: /Users/<USERNAME>/Containers/com.aerofly.aerofly-fs-4-mac/Data/Library/Application Support/Aerofly FS 4/addons/aircraft/c172
But for me it need to be in this one:
/Applications/Aerofly FS 4.app/Contents/Resources/aircraft/c172
Because there was no c172 in the aircraft folder. And the option file was missing the (2016) part
Now I think it should work in the future, thank you.