In أَمْكَنَكَ أَنْ تُوَاسِيَ, the person had the opportunity to offer support. After an, the imperfect verb tuwāsiya is in the subjunctive (manṣūb).
فَمَا آسَيْتَ joins the missed outcome with fa, while mā negates the action. The sentence therefore holds opportunity and failure to act in a compact contrast.
For example, someone could help a neighbor in need but chooses not to. This is precisely the moral gap the preacher highlights.