When configuring a project to use A360 content in AutoCAD Plant 3d 2017 or 2018, your setting will not save unless there is already plant content in that folder location. If the change option is available, and you select it, you will get asked if you would like to copy content to that location.
If it is not selected, you will need to copy the content using the Spec Editor. To do so, run the spec editor as an administrator.
Then go to Tools > Modify Shared content folder.
Then you set your path to the correct cloud folder, and make sure to check the Copy all content box. Click OK.
From that point, your project will be able to be set to use your A360 content folder.