Shalom,

The question of whether Zikah is from the Torah or not is a well-discussed topic. I will cite some sources you can look up:

a) The Rambam in the beginning of Hilchos Yibum (1:13) says Zikah is d'Rabanan.

b) The Rashba (Daf 27b) proves that Zikah is d'Rabanan as well.

c) The Steipler Gaon (in Kehilos Yakov, Yevamos #16) brings a number of proofs from different sources that Zikah in a few cases is from the Torah (Kidushin 4b with Rashi, Nedarim 74a, Sotah 18b, Tosfos on Yevamos 67b, DH Kinyan).

d) Rav Naftali Trop (Chidushei ha'Granat, Siman 12) says that Zikah is from the Torah, but only if the Yavam is Meyabem, then retroactively we know that the Zikah was there. He adds, too, that this is true only if there is only one Yavam.

I hope this helps,

Aharon Steiner