I think Eternallord66's answer is a good and valid interpretation and stands on its own. But, for my own edification, I'd also like to offer an alternative interpretation because I think there is room for a DM to rule both ways.
To begin with, the rules for a class are not written in the context of a multiclass character. The way a spellcaster + spellcaster multiclass works, the character's spell slots are not treated as belonging to either component class. Instead, the character gains a common pool of spell slots.
In the context of a feature like the Ranger's Primeval Awareness, this creates a conflict: the Ranger no longer has slots uniquely associated with that class. So either the feature stops being usable because the slots are no longer "Ranger" slots. Or, more reasonably, the slots are considered belonging to both classes.
In this case, features that specify slots associated with a particular class now apply to the multiclass slots.
With regard to a Warlock's pact magic, things are a little different because pact magic combines with other spell slots differently. To wit: they supplement, not combine.
However, the rules for multiclassing from a spellcasting class into Warlock (or vice versa) state:
If you have both the Spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the Spellcasting class feature, and you can use the spell slots you gain from the Spellcasting class feature to cast warlock spells you know.
So there is interplay with how the spells from either component class may be cast. If we step back from the rules and analyze them in a mechanics sense, we see that spells and abilities are both ways of producing some sort of unique effect through the game mechanics.
That is, using a class ability that says "you may expend a spell slot to [do something]" is functionally no different than casting a spell that achieves the same result. And if we are allowed to use a Warlock's pact magic spell slot to cast a ranger spell, it therefore stands to reason that we ought to be able to use that spell slot to pay for a class ability.
At the end of the day, the question being dealt with here is one of game balance. Leveling a character in one class and not another carries an opportunity cost that acts as a balancing force. A ranger gets 3rd level spell slots at level 9. The Warlock gets them at level 5. But in order to get those third level spell slots, they are giving up five levels of Ranger to get them: five levels that might have earned them those same level 3 spell slots in the first place.
However, it's worth pointing out that pact magic slots and true caster slots are not valued equally. Warlock spell slots, being a relatively rarer commodity, are designed to produce some pretty potent effects to compensate for their scarcity.
Warlock's Eldritch Smite, for instance, is more powerful than the paladin's Smite. Allowing the relatively more abundant true caster slots to fuel Eldritch Smite might tilt in the direction of being overpowered in the eyes of some people.
But building a character to exploit this synergy requires foregoing some quantity of true caster slots due to leveling up the slot-poor Warlock class instead of the more slot-rich true caster class. Does the opportunity cost offset? That's a question for the DM.
So, after all the pondering is done, I think Eternallord66's answer is absolutely supported by a strict reading of the rules. But I also think that a little looser interpretation supports the opposite conclusion: that if a class-specific spell slot is mentioned in that class's ability, it may not be unbalancing to allow any slot to be used for it and (because of how the multiclassing rules are written) and is arguably simpler to manage/track.
For what it's worth, DnD Beyond is no help. If you create a ranger/warlock character on the site, the digital character sheet shows ranger's Primeval Awareness but does not feature a "use" button prompting the player to select which spell slot to pay it with. The character sheet simply describes the Primeval Awareness ability and assumes/relies on the player manually ticking off the spell slot they want to pay it with.
There is one other caveat that I think is worth mentioning: Sometimes the rules impose limitations out of consideration for lore or narrative reasons. For example Druids can't use shields:
[...] shields (druids will not wear armor or use shields made of metal)
Pact magic slots are not of the same provenance, lore-wise, as other slots so there is a narrative reason to honor the distinction and prevent cross utilization. Then again, are a Celestial warlock's spell slots that much different than a paladin or cleric's?