Suppose that general normative requirement works like this: if X is generally required to A, this is partially constituted by X’s not-A-ing options in her choice situations starting out with a very high negative valence (that generally swamps any negative valence of the not-A-ing options). Now suppose that, in particular choice situation S, it is super-unlikely that X will pull off A-ing. In such a case, the relevant option is really her attempting to A. But also any attempt to A is almost certain to come to her not-A-ing. It seems plausible, then, that all of X’s options in S have nearly the same magnitude of highly negative valence. So there is not, as there would normally be, some huge “valence gap” between (token) A-ing and (token) not-A-ing. There is no normative “swamping” to leave A-ing as the far-and-away best option. And so, despite being under a general requirement to A, X is not, in S, required to A (realize this token of A-ing). 

(With respect to the general requirement, A-ing is still, barely, the best option, so, in this relative sense, X ought to (attempt to) A. And perhaps X is required to (attempt to) A relative to some distinct general requirement. Finally, perhaps X has strong reason to remove the barriers to her A-ing, so that she can be more responsive to the general requirement to A (this might correspond to X’s A-ing here being valuable or desirable for X). All of this is consistent with the requirement-blocking.)

Applied to Professor Procrastinate (individual moral requirement): she is not morally required, after all, to [accept and complete] (the assignment) – not required to keep her promise in the situation – despite being under the general moral requirement to keep her promises. For: whatever she does most immediately (accept or not accept), she has little or no chance of not doing something that does not constitute her breaking her promise. And so a nearly-equal promise-breaking pox is cast upon all her options, negating the conditions necessary for the specific normative requirement.

Applied to society and achieving social arrangements that realize full social or political justice, but that we stand little or no chance of realizing presently: we are not morally required to [implement and comply] (with the institutional principles or plans). For: all of the options stand little or no chance of realizing the general requirement and so a roughly equal social/political justice-y pox on them all. And so there is no option such that society is morally required to take it. (As indicated with normative requirement generically, this is consistent with all sorts of related moral requirements or moral reasons being present for society. In particular, it is consistent with the presently-unlikely justice-wise ideal social arrangement being highly morally valuable for society and with society having strong moral reasons to change social and individual-motivation conditions such that we no longer have little or no chance of realizing full justice in our social arrangements.)