# shellcheck foobar=baz
echo "Hello World"

Depends on your intention.

ShellCheck doesn't recognize the directive you're trying to use in a # shellcheck comment. See the Directives page for supported directives.

It could be misspelled, or you could be using an older version of shellcheck that doesn't support it yet.

None. If you wish to ignore this warning and continue without it, you need version 0.4.5 (commit 88c56ec) or later and a command grouping:

# Ignore an unrecognized directive in 0.4.5 or later:
# shellcheck disable=SC1107
{
  # shellcheck unrecognized=directive
  echo "Hello World"
}

Before 0.4.5, unrecognized directives are considered parse errors.