It is indexed (put site:https://www.functionaltennis.com/products/functional-tennis-practice-journal in google search and you'll see it there)
When I search for it your main site shows but the practice journal is shown as a site link below the main listing (see screenshot below) so it is being picked up by Google.
I'm not an SEO expert I'm afraid so other than editing your meta titles etc I'm not sure what would be a good next move. Hopefully someone a little more knowledgeable on that side of things will chime in ;-)
|2 hours ago|
|7 hours ago|