Why can't I find any products or collections by partial match using the search bar in Pre-Order Manager v2.0?

Unfortunately, partial matches aren't supported due to Shopify limitations.

Starting from December 16, 2019, our app uses Shopify API 2019-10, where requests that pass a value for the title match only if the value is the same as the complete title.

Example: Your product title is "Cherokee Boys Grey Melange Track Pants".
So you should search by the complete title "Cherokee Boys Grey Melange Track Pants" to get a result, not by a partial text "Cherokee".