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.