Only squares instead of letters and numbers are displayed in my calculator and other apps in Ubuntu 18.04

Correct font permission issues, then rebuild the font cache:

find /usr/share/fonts -type f -exec sudo chmod 644 {} \;
find ~/.fonts -type f -exec sudo chmod 644 {} \;
sudo fc-cache -r -v

It seems that you configured a Font which does not include the necessary glyphs to display. With a look to your name... do you use a Chinese font?

Maybe changing the font will help you. (see Font Settings Design)

The 3 line previous answer doesn't work on 20.04, or at least didn't on mine. It looks like they moved the folder. I used dconf reset -f / and just reset the desktop to default.