This problem does not occur on my PC version of Atom, but to get Emmet to work at all on macOS I had to make a simple edit to an Atom configuration file.

If you cannot get Atom to execute Emmet in the macOS version, go to the Atom menu and select Keymap. This will open the file keymap.cson, which, in most instances for newbies, will have examples that are all commented out. Insert the following code in this file:

'tab': 'emmet:expand-abbreviation-with-tab'

Save the file and close it. You will then need to close and restart Atom. This is the best fix I have found in my research of this issue.