Hello there SpaceMan,
Thanks for posting in the Apple Support Communities!
For clarification, you're able to complete the purchase using Apple Card Monthly Installments on your Mac by going to the website, but unable to do this via the Apple Store app on your iPhone, right? If that is correct, let's be sure you have the latest iOS version installed on your iPhone to see if that could be related to what is occurring. Be sure to create a backup before installing an update if one is available. For assistance with this, check out the following steps which can be found here: Update your iPhone, iPad, or iPod touch
Update your iPhone, iPad, or iPod touch wirelessly
1. Back up your device using iCloud or your computer.
2. Plug your device into power and connect to the internet with Wi-Fi.
3. Go to Settings > General, then tap Software Update.
4. If you see two software update options available, choose the one that you want to install.
5. Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now. If you don't know your passcode, learn what to do.
You might see that your device is up to date and that there's an upgrade available. For example, even if your device is up to date with the latest version of iOS 14, there might be an option to upgrade to iOS 15. You might want to upgrade iOS when you pair an accessory with iPhone.
If an update isn't available, let's confirm there's enough available credit on your Apple Card to rule that out as being a possibility. Although you'll be paying monthly for the purchase, you would still need the full amount available on your Apple Card. Check out more information on this via the following resource: How to view and pay Apple Card Monthly Installments
What you need
An Apple Card account with enough available credit to cover the cost of your new device.
Make sure your device has the latest version of iOS or iPadOS, or macOS. On iOS 13.1 or earlier, you can't see your installment details and Total Balance includes the total amount you finance.
Apple Card Family participants can make a purchase using Apple Card Monthly Installments if the account owner or co-owner has not set a transaction limit for the participant. If the account owner or co-owner has set a transaction limit for the participant, it must be at least equal to the total purchase price of the eligible Apple product. This includes tax and shipping.
If that isn't related to what you're seeing either, Contact Apple Support to look into this in more detail.
Have a great day!
