Is there a single-word antonym for "Opportunity"?

In business management context, the antonym of 'opportunity' is 'threat'. An 'opportunity' by definition is a circumstance where one has possibility of gaining some benefit, and a 'threat' is exactly the opposite - a circumstance where one has possibility of some loss.

However, in day-to-day context, we could take 'adversity' as antonym for 'opportunity'. 'Adversity' here meaning an unfavorable condition/circumstance, whereas 'opportunity' meaning favorable condition/circumstance.

Dead-end is used as both adjective and noun to convey the notion of lack of opportunity. (The Free Dictionary).

To the extent that you’re also considering antonyms for “opportunity” as well as terms describing the absence thereof (e.g., my earlier suggestion of “dead-end,” above), the words ‘opportunities’ and ‘pitfalls’ are sometimes juxtaposed to show contrast (if not opposition), especially when discussing those that are hidden (although slightly redundant with 'pitfall') and/or potential.

Pitfall/noun = a danger or problem that is hidden or not obvious at first (Merriam-Webster)

And to add one more suggestion for the “absence of opportunity” angle of your quest to go with “dead-end,” since many opportunities are unfortunately ‘wasted,’ perhaps a “wasteland” could describe the place or state where these wasted opportunities go to languish alongside those that never were.

Wasteland/noun/3 = something, as a period of history, phase of existence, or locality, that is spiritually or intellectually barren. (Dictionary[dot]com)