The reason you are having issues is the fact you have no reference bodies in your supplied file. If you notice, as I point out in my Screencast, that big read square at the beginning of your timeline means you have real issue with this model. You were working with a "cached" body only. I recovered your model using the Insert Derive command into a new file. This process removes all the links, bad ones in your case, from the new model body. I can then perform the same technique I showed you yesterday to change the one side of your body you call a door into a separate body. You can do what you wish with the body when finished. My model is attached which is the recovered file.
Since you are a beginner in Fusion 360 you must remember model required history hierarchy features and bodies must be maintained. For example, if I have a block and I put a hole through it, I cannot get rid of the block because the hole depends on the block being there. Something similar happened to your model. You deleted something from your timeline that was required for the body you were currently working on. There are two options, when you click on a body in the Browser, Delete and Remove, learn to use Remove more than Delete. Remove essentially deletes the body, as far as you are concerned, but leaves the necessary "data" in the model to support dependent features.
John Hackney, Retired
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