The object you have selected in your screenshot is not a 'Line'. 'Schraffur' = Hatch??

Do you actually have any 'line' objects or is it just a hatch without a boundary?

If it is just a hatch, you can recreate the hatch boundary as a polyline. See here.

If you already have lines and they just don't want to join, check do the ends of the lines actually coincide exactly? If not, you can adjust the "Fuzz" value when you Join in PEDIT command. (At least, this option exists in AutoCAD for Windows, so i think it will also exist for Mac version)

If you need further help, we probably to know:

• what is your AutoCAD version (2017, 2018, 2019 etc)
• what exactly happens when you try to use the command? Do you get any error messages? What does it say in the commandline at the bottom of your screen?
• can you share the geometry (just the lines you are trying to join) so we can experiment on our own systems?