Why can't I download Ubuntu system?

Website stated you can use Ubuntu but you cannot

Dear friends
The question I received here is that you cannot use the Ubuntu system, right?
How did you download this system?
FriendlyCore / Debian / Ubuntu / OpenWrt / DietPi are all based on a same Linux distribution and their installation methods are the same.
It need to Flash to TF, and using TF card run the Ubuntu system.

This is our wiki: Naopi R4S. There are detailed teaching plans above, thank you

The marked system is based on ubuntu, you can download the mirror from here.

For Ubuntu core , you can reference this https://ubuntu.com/core