Star Trek: Captain's Chair · Board game rules
How do mandatory costs work in the game?
Quick answer
All effects of a resolved operation are mandatory unless preceded by "may"; if a required cost (spending resources, discarding, exhausting, etc.) can't be paid, you simply cannot resolve that operation at all.
Details
When you choose to resolve an operation, every effect it lists must be carried out fully unless the text explicitly says "may." Costs are the specific parts of an operation you must pay to get its benefits — these include spending resources, putting/discarding cards into specific places (Discard pile, top of deck, Staging Area, etc.), or exhausting a card [p.10 — ACTION STEP]. If you're unable to pay a required cost, you simply cannot choose to resolve that operation in the first place [p.10 — ACTION STEP].
This "cost" logic also governs individual keywords: Discard is always treated as a cost, so if you can't discard a matching card, you can't trigger that effect [p.29 — DISCARD]. Similarly, Put a card (somewhere) and Exhaust are also considered costs — if you don't have an eligible card to put or exhaust, you can't resolve the operation requiring it [p.31 — PUT A CARD; p.30 — EXHAUST]. Spending resources works the same way: return the required tokens from your resource pool to the supply; if you can't spend the full amount, you cannot resolve the operation, though Glory can substitute for Dilithium (1-for-1) or Latinum (1 Glory = 2 Latinum) [p.8 — SPENDING RESOURCES; p.32 — SPEND RESOURCES].
By contrast, non-cost effects that turn out to be useless (e.g., a "benefit" you can't actually apply) are simply skipped rather than blocking the whole operation — this distinction between cost and non-cost effects is what determines whether an unresolvable part stops you from playing the card at all, versus just being ignored [p.10 — ACTION STEP].
Watch out: In a "do A to do B" sentence, A is treated as a cost for B — so if you can't do A, you can't resolve B either, even though B might otherwise look like a normal effect [p.10 — ACTION STEP].