Can I use the Apple Pay Later for balance on the Apple account I already have?

Sorry, your question is not very clear.

Apple Pay Later issues a loan to you, you can then use to make purchases, and pay in installments back to Apple.

If you already have balance on your account, then you just use that to make the purchase, but don't get the installment option.

You can use Apple Pay later for your installments, and then use the balance on your account to pay back Apple during the installments, but you'll still receive the loan and the loan is what all be used to pay the merchant, not your account balance directly.

click here ➜ Apply for an Apple Pay Later loan and make a purchase - Apple Support