I can't find Japan Transit Planner in my App Store

Usually, an app such as this one is only offered in the local App Store. This app is likely only available to a user in Japan with a registered Japanese Apple Account.

M1 Mac mini, macOS 15.6.1;  Watch, Series 10 GPS-only, watchOS 11.6.1; iPhone 16, iOS 18.6.2; TV 4th Gen, tvOS 18.6 HomePods, audioOS 18.6

It would be highly unusual for an app to be written in English if the only ones who could get access to it were those who had Apple accounts registered in Japan (i.e., namely native Japanese speakers who may not know a lot of English). Making it available to a wider audience would promote travel in Japan. As I expect:

I am in the USA. I just did a search for "Japan Transit Planner" on my iPhone and it is second down in the list of offered downloads.

Hard to help you without knowing where you are located. Not all apps are available in all stores.

Hello~ It’s also available on my iPad here in the USA…

~Katana-San~