When switching between master layouts for onMessage pages, the layout being switched to must have an equal or greater number of Page Content widgets in place than are being used on the page that is having its layout switched. For example, if a page has three page content regions in use, it won't be able to be switched to a layout that has two or fewer Page Content widgets.
There are two options for adjusting the page to allow the layout to be switched successfully.
Option 1: Add Page Content widgets to the desired master layout until there are an equal number of areas for page content as are being used on the page having its layout switched.
Option 2: Remove all content from page content regions on the page being switched until there are an equal number in use as exist in the desired layout. Rows created within a page content region do count as the region being in use, so if the page currently has three page content regions with content in them and the layout is being switched to a layout with only two Page Content widgets, all content (including created rows) need to be relocated or deleted from one of the page content regions so there are just two page content regions in use that will need to be moved into the new layout's page content regions.