You are attempting to compare one multi-page PDF to another, but the Multi-page Comparison option is not presented to choose Setup Comparison for: All pages. Or, if you are successfully able to select All pages the toggle from one page to the next does not keep the common pages in synchronization with one another making it impossible to Compare.
Attempt to open the Page List panel fails:
One or more of your PDF files have been the Prepare for Viewer task.
Delete View data for the PDF files to be compared and now Multi-page Comparison option will be presented and allows to toggle through the various pages while rendering the chosen Compare view options.