Can you cast a face-down creature spell using morph even if you don't have enough mana for its morph cost?

Morph allows you to cast a creature card face-down as a 2/2 colorless creature for 3 generic mana, but that cost of {3} is still a real mana cost that must be paid. CR 702.36b states that casting a card using morph means you follow the normal rules for casting a spell, except that it is cast face-down and its cost is replaced with {3}. CR 601.2f requires that you have the ability to pay all costs before you can legally cast a spell.

So the morph cost of {3} is not optional or free — it is the alternative cost you are paying instead of the card's normal mana cost. If you cannot produce three mana, you simply cannot cast the card face-down via morph, just as you couldn't cast any spell whose mana cost you can't pay.

Note that some cards have a megamorph or other variant ability with different costs, but the same rule applies: the cost listed must be payable. Additionally, abilities that reduce costs (like cost reducers) can apply to the morph cost, potentially making it cheaper, but they cannot make it free unless they specifically say so.

Example: You have only 2 lands untapped and a creature with Morph {3} in your hand. You want to cast it face-down. You cannot — you need at least three mana to pay the morph cost. Even though the card's printed mana cost might be higher, {3} is still the minimum you must pay to cast it face-down via morph.