Problematic code:
echo ‘hello world’
Correct code:
echo 'hello world'
Some software may automatically replace your regular quotes with slanted Unicode quotes. Try deleting and retyping them, and/or disable “smart quotes” in your editor or OS.
Exception:
If you want to use typographic single quotes, you can put them in double quotes (or typographic double quotes in single quotes) to make shellcheck ignore them, e.g.,
printf "Warning: ‘wakeonlan’ is not installed.\n"