Unable to install apps on new iPhone 11 Pro Max because I have to purchase them first
You just need to delete the app (or, more specifically, the icon placeholder for the app) from your iPhone's Home screen. Then when you go to the App Store again you should get the cloud download symbol indicating you've previously purchased the app. Tap on that and it should download just fine.

To delete apps from the Home Screen on iOS 13, follow these steps:


On your iPhone's home screen, long press on the app icon and let go
From the options that appear, tap Rearrange Apps
Now you'll see the familiar jiggle action and the associated cross at top left of the icon
Tap on the cross to delete the app
Now proceed to the App Store to download it again