English: "The festival of Sukkos you shall make" — "you shall make" and not from that which is already made; from here they said: if one trained a vine or a gourd or ivy over it and then placed valid sechach on top, it is invalid.
On the daf / in context: The sukkah must be valid at the moment of its construction; one may not rely on later improvements to cure an initially invalid structure.
Type of Learning
Derash on the verse (Sukkah) — a sukkah must be made, not merely found already made.
What Is Learned
A sukkah that was initially invalid (for example, covered only by a living vine) remains invalid even if valid sechach is later added on top.