This is a unicode quote. Delete and retype it (or quote to make literal).

Problematic code:

echo ‘hello world’

Correct code:

echo 'hello world'

Rationale:

Some software, like OS X, Word and WordPress, 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.

Exceptions

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"