My purchase was supposed to be pay in monthly installments but it’s showing as pay in fill

What you expected and how an installment plan works are two entirely different accounts.

When you buy a house, car, Apple iPad etc. the entire amount of purchase is charged to you. You pay the monthly installment which in case of the house and car, include interest for borrowing the money to pay for the house and card.

The Apple Installment Plan is a zero interest payment plan/arrangement. In essence you’re borrowing the money for free. You pay your amount every month and when you complete the payment plan you’ll have borrowed the money for free. That’s a great thing to be able to buy something and not pay interest.

When you use a credit card to make a purchase, you are borrowing money from a bank. In the case of Apple Card MasterCard, you are borrowing money from Goldman Sachs Bank. Goldman charges interest on your Apple Card account when you don’t pay it monthly. You are in an interest free installment plan the allows you to borrow money from Goldman for free. That’s a pretty cool thing.

Your credit limit on your credit card is the bank money, not yours. You pay interest when you borrow/charge your account. Money on your debit card is you money, not the banks, two entirely different things.

Apple Card - Monthly Installments - Apple

Products you can buy from Apple with Apple Card Monthly Installments - Apple Support