SC2204

if ( -d mydir )
then
  echo "It's a directory"
fi

if [ -d mydir ]
then
  echo "It's a directory"
fi

Tests like -d to see if something is a directory or -z to see if it's non-empty are actually flags to the test command, and only work as tests in that context. [ is an alias for test, so you'll frequently see them written as [ -d mydir ].

( .. ) is completely unrelated, and is a subshell mostly used to scope shell modifications. They should not be used in if or while statements in shell scripts.

If you wanted to test a condition, rewrite the ( .. ) to [ .. ].

None.

This error is triggered by having a unary test operator as the first command name in a subshell, which won't normally happen. Note that there's a similar warning SC2205 with a higher false positive rate.