I have a MacBook Air 11" and a MacBook Air 13": the 11" is lot older than the 13". Although High Sierra is installed on both MacBooks, on the 13", I can't find caps lock option in Settings-> Keyboard-> Input Sources, but on the 11" it is present. I even don't know what to search for: both laptops have the latest update installed from the App Store.
+Go to Apple > About this Mac and double check that the machine is actually running 10.13 and not 10.12 (caps lock does not work for Russian in 10.12).
+Add an Arabic keyboard to your input sources list and restart.
+Do a safe boot
+Set up a new user account and see if you have the same problem when logged into that.