Hi Lutzkie,
There are a number of resource stations that cannot be placed on flooring/foundations at the moment. Players usually place far plots on the ground, however there are some workarounds that players are using that may be benifical to you, but it may require resources to build around your farm area that you can then break back down afterwards.
Please see the known issues report on this for potential workarounds: [Known Issue] Difficulty placing crafting stations inside buildings