Hello Massimo Romano,
The supply voltage can be either 1.8V or 3.3V. However, 1.8V operation is limited by speed by Erratum 010450 (you can find it on the link below) so you would need to use 3.3V.
https://www.nxp.com/docs/en/errata/IMX6ULLCE.pdf
I hope this helps!
Regards,
Thanks