Xcode 12 Unable to use Interface Builder to add subview to scrollview
I have the same issue on Xcode 12.0.1. As a workaround you can start by setting up the root view of your scroll view content, select it and use the Editor / Embed in / Scroll View
command (or alternatively, the last bottom right icon in the canvas). Then you can add to the root content view normally by drag & dropping.