How do I put a dot under a letter?

Control + command + spacebar opens the character viewer from the current application. What you are looking for is the combining dot below, which when entered immediately after a letter, will put the dot below that letter. ḥ .

There is a search window in the character viewer. Use it by entering combining dot. The resulting, second dot in the center panel is the one you want, and double-clicking that character immediately after the letter you want to place it under will do the trick, as I demonstrated with the h character above.

To see a list of all combining characters, just enter combi in the character viewer's search field.

One way is to use the Character Viewer > Latin… ḥ

FWIW, The character Viewer is available at least as far back as OS X 10.9 Mavericks. I can't check before that.

You're welcome. High Sierra does. System Preferences > Keyboard

Hmmm. There should be. You could try resetting NVRAM.

Reset NVRAM or PRAM on your Mac - Apple Support

If that doesn't work, try creating a new user to log into.