Which is the policy adopted by Community background process regarding to the reviews approval? I do not understand why a review I made on spelling and code formatting has been rejected.
Users who have enough reputation to review edits can review edits.
It takes two such people to reject an edit - they will normally explain their (unrelated) reasoning in the rejection.
Also see How do suggested edits work?
Looking at the rejection by "community" that @jadarnl27 linked to in the comment, I would say that your edit was superseded by a more significant edit by another user and the system took that one, rejecting yours in the process.