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The wiki states that you get the achievement for buying decorations, no matter how often you place them.
https://wiki.diggysadventure.com/index.php?title=Achievements#Aesthetician
So in order to get on you have to buy more.
The placing in your camp has nothing to to with it. As in every achievement it is getting the things, not what you do with them afterwards.
So get and buy new ones, then the number should go up.
To remove or sell decorations, check your settings if the decoration lock is off = red.
Selling doesn’t reduce your achievement. It gives some coins and you have space to place other decorations.
If you tap on a decoration in your camp, the dollar symbol means sell and the arrow down means back in your backpack. Both do nothing with the achievement.