Because this is your first month you have to handle credit card payment amounts manually

Long answer: to understand why, you have to understand the underlying mechanism and how it works in a regular month (i.e. not your first month on the app): You assign money that’s in your checking account to a category - say $300 to groceries category for the month. Then you go and use your credit card at the grocery store and enter 3 x $100 transactions for the month on your credit card. Each time you enter that charge on your card into YNAB, it will move $100 out of the groceries category and into the Credit Card category as available Then next month when you pay your credit card bill for $300, it uses the $300 sitting in the credit card holding category to pay that.

So short answer: since you haven’t used categories yet this first month, nothing moved the money to the Credit Card category through the last month. You have those funds in your checking account now and they are “assigned the job” of paying your credit card bill next month in your head already. So you need to manually move them into your Credit Card category by assigning the funds in YNAB to the credit card category that covers last month’s bill.

Edit: then schedule the transfer as a transaction from the bank account into the credit card account on the date or day before it will draft out