A saving throw—also called a save—represents an attempt to avoid or resist a threat. You normally make a saving throw only when a rule requires you to do so, but you can decide to fail the save without rolling. The result of a save is detailed in the effect that allowed it. If a target is forced to make a save and lacks the ability score used by it, the target automatically fails.
Page 373 of PHB 2024
Bolded section is what I am concerned with. Does this mean that if I am a Barbarian with Strength/Constitution saving throw proficiency, that I automatically fail when needing to perform a Dexterity saving throw (because I do not have the ability score used by it)? Example here would be the Fireball spell cast by an enemy.
A lot of people are noting ability score of 0, but I think the wording was meant to be applied to more inanimate things. An ability score of 0 is just a -5 to the score. Lots of sentient magic items and objects will have limited stats. For instance, Whelm is a magic sentient warhammer with an Intelligence of 15, Wisdom of 12, and Charisma of 15. If, for instance, it was forced to make a Strength saving throw, it has no Strength score, and therefore fails. Likewise vehicles often have Strength, Dexterity, and Constitution scores, but no mental scores. If you forced it to make a mental save, they would automatically fail.