Can you buy bitcoin with Apple Cash?

Does CoinBase actually provide an “ApplePay” button on their online “purchase” screens?

CoinBase also might not be allowed to operate in your particular state of residence.

See: https://www.coinbase.com/legal/licenses

Or, it might be getting cancelled by the AppleCash issuer … Green Dot Bank. (Apple’s not involved in actual transaction processing)

Suggest that you contact Green Dot to ascertain what’s going on.

Contact Green Dot Bank by calling (877) 233-8552, by mail at P.O. Box 5100, Pasadena, California 91117, or visit www.greendot.com.

Source: AppleCash T&C (accessible from Wallet)

Apple Cash is recognized by the banking system as a Debit Card. So anything you can buy with a debit card can be bought with Apple Cash. You would need to contact Coinbase for your answer.