If the template has disabled one or more define pages, you will need to remove the code that does that from whatever file it is in. Look at the files in your custom template folder, especially /common/tpl_main_page.php and any files with "index" in the title. Compare them with the /template_default/ versions of the same files, and you may see the location(s) to fix.
You may want to switch to the Classic template and verify that the define pages work there.






