How can I get assistance with my Apple Card?

You might want start by stating what the problem is. There are a number of people here who know a lot about Apple Cards.

If you're asking about the Apple-branded Mastercard, see also the contact info here:

How can I contact Apple Card support? - Apple Community

CONTACTING — Goldman Sachs Bank 

• Using Messages; chat with an Apple Card specialist

• Call toll-free at 877-255-5923

• Write to: Goldman Sachs Bank, Lockbox 6112, P.O. Box 7247, Philadelphia, PA 19170-6112.

• https://www.goldmansachs.com/terms-and-conditions/Apple-Card-Customer-Agreement.pdf

I can't use my card; it shows no outstanding balance but displays a zero available balance.