Can you cast a spell for its escape cost if you don't have enough cards to exile from your graveyard?

Escape is a keyword ability that allows you to cast a spell from your graveyard by paying an alternative cost — but that alternative cost includes both a mana payment and exiling a specific number of other cards from your graveyard. Both parts of the cost are mandatory. (CR 702.156a)

When you cast a spell for its escape cost, you must be able to pay the full alternative cost at the time you choose to cast it. If the escape cost requires exiling, say, three cards from your graveyard, and you only have two other cards there, you do not meet the requirements and simply cannot cast the spell using escape. (CR 601.2b, 601.2h)

This is similar to how you cannot cast a spell if you cannot pay all of its mana cost — partial payments are not allowed. The escape cost is treated as a single alternative cost package, and you must satisfy every component of it simultaneously. (CR 117.3)

Concrete example: You have Uro, Titan of Nature's Wrath in your graveyard. Its escape cost requires paying 3GU and exiling five other cards from your graveyard. If you only have four other cards in your graveyard, you cannot cast Uro via escape — even if you have the mana available — because you cannot exile the required five cards.

If you want to cast the spell, you must wait until you have enough cards in your graveyard to meet the exile requirement, or cast the spell through some other means if available.