Turns out the tiles must be empty and that includes non-tilled soil if you want to build a coop or other farm buildings. Seasons just changed and I don't want to wait ~20 game days for them to change again. Is there a way to make the soil non-tilled faster?
Stardew Valley – How to Un-till Soil
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Best Answer
To un-till soil just hit it with your Pickaxe.