“Opportunity” by Walter Malone
As you might observe from some of my previous posts, I like to keep things in context; so, before addressing your question about the last two lines of this poem, I find it prudent to unpack the previous lines first for sake of continuity.
They do me wrong who say I come no more
When once I knock and fail to find you in;
In context in ‘The Fall of the House of Usher’, the poem ‘The Haunted Palace’ is a conceit (an extended metaphor) in which the palace represents Roderick Usher’s head, and its occupants his thoughts. In the story, the poem is introduced by the narrator in a manner that recommends this interpretation to the reader:
The words of one of these rhapsodies I have ...