Q: Can I get my money back?

Developers have a choice of creating a universal app that will work on both iPads and iPhones or they can make separate apps, one for the iPhone and one for the iPad. If they chose to make separate apps, they may also charge separate fees. When you went to download the app for your iPad, did it have a little cloud with an arrow? Or did it have a price?

If it had a price then it was most likely a separate iPad version of the app for which the developer is charging you. You can check by looking at the icon on the apps page in the App Store. Does it have a little + in the corner? If not, it's not a universal app.

What is the name of the app?

If they have no plus, they are not universal apps and the developer has apparently chose to charge you separately for both apps.

What is the name of the app?

It appears that the iPhone version is "Procreate Pocket", an separate app. It costs 2.99 USD

"Procreate" is, according to the developer, "only available on iPad" and is 5.99 USD.

Things could have changed.

The bottom line is no, you can't get your money back.

Sorry.