Rune of Mortality — ruling 1

Runes can target and be attached to any permanent, even one that isn't currently a creature or an Equipment. You can cast a Rune choosing a Vehicle as the target, for example. The Rune's enters-the-battlefield ability will trigger, and you'll draw a card. The Rune won't do anything while the Vehicle isn't a creature, but if it becomes one later, the appropriate ability will start applying.

Rune of Mortality — ruling 2

If the target of an Aura spell is an illegal target as that spell tries to resolve, it won't resolve, it won't enter the battlefield, and none of its enters-the-battlefield triggered abilities will trigger.

Rune of Mortality : règles, fonctionnement et interactions (MTG)

Rune of Mortality 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

Enchant permanent When this Aura enters, draw a card. As long as enchanted permanent is a creature, it has deathtouch. As long as enchanted permanent is an Equipment, it has "Equipped creature has deathtouch."

How it works : Rune of Mortality

Original English text — Wizards of the Coast / Scryfall

#1February 5, 2021· Wizards of the Coast
Runes can target and be attached to any permanent, even one that isn't currently a creature or an Equipment. You can cast a Rune choosing a Vehicle as the target, for example. The Rune's enters-the-battlefield ability will trigger, and you'll draw a card. The Rune won't do anything while the Vehicle isn't a creature, but if it becomes one later, the appropriate ability will start applying.
No other card is named in this ruling.
#2February 5, 2021· Wizards of the Coast
If the target of an Aura spell is an illegal target as that spell tries to resolve, it won't resolve, it won't enter the battlefield, and none of its enters-the-battlefield triggered abilities will trigger.
No other card is named in this ruling.