Oh I think it's a feature in Valheim Plus. towards the top of the config page is feature that removes the invalid placement error for a lot of things. Pretty sure this is what allowed it.
I'm not sure what's wrong, so I'll go with a detailed explanation and hopefully the answer will be within. ok so after downloading the BuildShare mod and installing it, it will provide you with a folder named "BuildShare" in your main Valheim folder. My building file should be saved into the "Builds" folder inside that. Once the file is there you can load up the game and on the left side (unless you customized it) of the screen you will see the BuildShare interface icon. Clicking that should bring up the menu, where you will want to navigate to the Build tab (you can also use the directory to make sure the file can be seen). After getting the area ready for it you should be able to use the "Toggle" feature to make a ghost show up to help you get it in the right position. Once your happy with that you should be able to click build to create it.
if this doesn't help I can try and provide more info.
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Found it: https://www.nexusmods.com/valheim/mods/854?tab=description
if this doesn't help I can try and provide more info.