Can you put a card exiled with Suspend onto the battlefield if it has no mana cost?

Suspend works by eventually letting you cast the suspended card without paying its mana cost. The key rule is CR 702.62d: when the last time counter is removed from a suspended card, its controller may cast it without paying its mana cost. This is explicitly a 'cast without paying mana cost' effect, not a 'put onto the battlefield' effect — so the card still goes on the stack and resolves normally.

If the card has no mana cost at all (like a card with only Phyrexian mana or a card that simply lacks a mana cost in its upper right), that is fine. CR 702.62d grants the permission to cast it without paying its mana cost, meaning you pay nothing. A missing mana cost does not prevent the cast — it just means there is nothing to pay even under normal circumstances.

One important nuance: if the card is not a land (which cannot be cast), it goes on the stack as a spell and resolves onto the battlefield (or wherever appropriate). If it somehow were a land, suspend's cast instruction would fail, but lands do not have mana costs anyway, so this edge case is handled separately by CR 702.62e, which says if you cannot cast the card, it stays exiled.

Example: You have a suspended copy of Living End, which has no mana cost. When the last time counter is removed, you cast it for free via suspend. It goes on the stack, players may respond, and it resolves, triggering its effect — exactly as if you had cast it through its cycling cascade chain.