وَمَنْ جَامَعَ فِي الْعُمْرَةِ قَبْلَ أَنْ يَطُوفَ أَرْبَعَةَ أَشْوَاطٍ أَفْسَدَهَا
Translation: Whoever has intercourse during Umrah before completing four tawaf circuits invalidates the Umrah.
The decisive threshold for Umrah is four completed circuits of tawaf. If intercourse occurs after only three circuits, the text deems the Umrah invalid.
This is a rule concerning intercourse during Umrah, not simply a ruling on forgetting to perform tawaf. The following clause explains what remains obligatory.