Seems to be impossible to place the Land Based Supply Camp despite being on flat solid ground. I am using MineColonies in the latest version of the Valhesia 3 modpack, so I suppose there could be a conflict that I am unaware of. This is what I'm seeing though -
Flat Solid Ground -

Schematic for Supply Camp placed -

Result when pressing the checkmark -

I've tried this in Survival and in a Creative Superflat World just to make sure I was not doing something wrong in Survival.

Nevermind, apparently it has to be placed in the ground. That is very unclear from the error message, message says it has to flat solid ground. Doesn't say it has to be placed in the ground