You can cast Rat Out without a target just to create a Rat token. However, if you do choose a target, and that target is illegal at the time Rat Out tries to resolve, the spell won't resolve and none of its effects will happen. You won't create a Rat token.

Rat Out : règles, fonctionnement et interactions (MTG)

Rat Out is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Up to one target creature gets -1/-1 until end of turn. You create a 1/1 black Rat creature token with "This token can't block."

How it works : Rat Out

Original English text — Wizards of the Coast / Scryfall

#1September 1, 2023· Wizards of the Coast
You can cast Rat Out without a target just to create a Rat token. However, if you do choose a target, and that target is illegal at the time Rat Out tries to resolve, the spell won't resolve and none of its effects will happen. You won't create a Rat token.
Card named in this ruling
Rat