Can I test a product page against a regular page, a PageFly page or a landing page? (Essential CRO Split AB Testing app)

No. Both versions in a template test must be the same type of page, for example two product templates, two page templates or two collection templates. The app won't start a test that compares, say, a product template with a page template.

If you want to compare two designs, create both as the same page type. For example, duplicate your product template and rebuild the landing page design inside it, or test two page templates against each other.