Create a new user just to try this out.
Then, log out of your current user, log IN to your new user.
Test the keyboard text replacement. On my way! would be a good test. (It changed instantly for me
after typing the letters [o.m.w]
Make sure that your "Region" in the Language and Regions pref pane is set for United States, and that the Preferred Languages list has English at the top. You can also try deleting any other languages showing by selecting, then click (-) to remove.
Make sure that your keyboard (Keyboard pane/Input sources tab) is set as a U.S. keyboard, and not something else like an extended unicode setting.
Finally, go to the Keyboard/Text pane again. Change the
Spelling preference from "Automatic by Language" to "U.S. English".
BTW, your title confused me into thinking at first that you were having a (hardware) problem with a replacement keyboard.
You might have a better thread title, like "Keyboard
text replacements not working"