I have used RasPi Yunohost of a long time. However I have bought a Udoo x86 and installed Lubuntu Jessie. However yunohost still has an error during installation, saying it requires debian jessie to be installed but Lubuntu is based on debian jessie, so I am unsure of this issue.
Well, your question is legitimate If Lubuntu is based on Debian, so why can’t YunoHost be installed on top of it ? The answer is that Lubuntu (and any Debian-based distro) already tweaks Debian its own way, which means that if you installed YunoHost on top of it, it would probably not work because the system won’t be in the state that YunoHost expects.
(For the same reason, you cannot for instance install Lubuntu on top of YunoHost)
So basically, the easiest thing to do would be to reinstall a clean Debian and install YunoHost on top of it. (Or you can use the pre-installed Raspberry Pi images)