Error while using Pane Layout and Header Content layour

Its strange. Two things to check

1. What are the components you have inside the collapsed pane layout.

2. What version of Appian are you in?

Also try adding an empty pane layout inside the contents and see it if errors out too.

 It's not gonna accept. Because both are outer shells, Clearly mentioned in the documentation below . 

https://docs.appian.com/suite/help/24.1/Pane_Layout.html#panes-as-a-top-level-layout

In the new update, it will accept.

From 24.3 version, pane layout is supported within header content layouts. Please see the release note for reference:

https://docs.appian.com/suite/help/24.3/Appian_Release_Notes.html#interfaces