A Mendix Placeholder can only be used in a Layout to define where page content goes. It is not a normal widget, so it cannot be placed inside another widget’s slot that allows child widgets. When you try to put a Placeholder into a pluggable widget slot, Studio Pro checks the rules and throws the C6700 error. You could try keeping the Placeholder in the Layout and placing your pluggable widget inside that Placeholder instead. If you need a reusable structure with editable content, use Layouts + Placeholders, and treat your pluggable widget as a normal widget inside the layout, not as something that contains a Placeholder itself.