Hello Melicy,
If the keypad on your iPhone is not working as expected, we'd like to have you try a few steps for us.
First, let's restart your device and then test to see if the issue remains. Some times just restarting can clear up things like this when they occur. Also, check to make sure that you're running the latest version (15.5) of software. If not, go ahead and update and test to see if the issue persists. The resources below will guide you through those steps, if needed.
Restart your iPhone - Apple Support
Update your iPhone, iPad, or iPod touch - Apple Support
If the issue remains after trying those steps, can you provide a little more details? Does this only happen in the App Store? Or does this happen when trying to make purchases in other apps as well?
Have a wonderful day!