Using the function keys on an A1048 keyboard

Just to add, when you look on your keyboard there is nothing printed on the top of the function keys like more modern Apple keyboards…

So you have nothing to choose from.

Since you don't have the checkbox to set the Function keys

you may have to use the Function key (Fn) in conjunction with the Function keys to get that key to work.

However, if you're running 10.13.6 as your profile states then you should have that checkbox. So do the following:

1 -boot into Safe Mode, Mac OS X: Starting up in Safe Mode, check the preferences and Function keys from there.

2 - log into another user account and check the Function keys from there.

3 - download and run Etrecheck. Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

You mean this?

The reason you don't see that option is because your model of keyboard does not have switchable function keys.

If you go to the menu bar, and show Keyboard Viewer, when you press the function keys on your keyboard do the function keys highlight on the Keyboard Viewer? If not, your keyboard may not be 100% compatible.

Just to add, if the keys do highlight, certain apps may have function keys preassigned, and you can also create your own shortcuts. macOS Sierra: Create keyboard shortcuts for apps

The plus sign only shows when you select App Shortcuts. You have Screen Shots selected in your post.

You're welcome.

You're welcome. I hadn't thought of it right away.

Once Apple started assigning special functions to the Function Keys, brightness, media keys, etc., they added the option to switch function key priority.