And when I try to diagnose, the spell check dialog box disappears as soon as I close the “spellcheck is complete” pop-up.
What are your language related settings in Tools > Options > LibreOffice >Languages?
Screenshots would be helpful.
You might be missing the spell checker backend named aspell. Assuming that your OS is Linux, or similar.
- You don’t actually need aspell, you can install an extension.
- I guess, the asker has figured out something since 2015