How do you enable key repeat?

When I hold down a character on my keyboard, I want that key to repeat. For some characters, like space, it does. But for other characters, like a, repeat does not work. Instead, an overlay displays showing the character with lots of different accent marks. How do I make key repeat work properly for all characters?

NB https://support.apple.com/guide/mac-help/set-how-quickly-a-key-repeats-mchl0311bdb4/10.15/mac/10.15 says that if I set key repeat to off, the accent mark overlay won't display, but that is not true. The accent marks still display, and keys still do not repeat. (I mean, I wouldn't expect that key repeat OFF would mean keys repeat, but that's what it says.)