I can't set up Apple Cash as my payment option

See -> Set up Apple Cash

Set up Apple Cash - Apple Support

• Be at least 18 years old and live in the United States.

See -> Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

• Apple Cash only in the USA

I want to make Apple Pay my default payment.

Apple Pay is the way to pay. It replaces your physical cards and cash with a method of payment that is private, easier and safer. Whether you pay in a store, on a website or in an app. This is the new payment.

it is actually the same as paying contactless with your bank card, only via the iPhone.

See the following Apple articles:

Use Apple Pay for contactless payments on iPhone

Use Apple Pay for contactless payments on iPhone – Apple Support (UK)

Learn where to use Apple Pay

Learn where to use Apple Pay - Apple Support

Make purchases using Apple Pay

Make purchases using Apple Pay – Apple Support (UK)

Set up Apple Pay

Set up Apple Pay - Apple Support

• add a credit, debit, or prepaid card to the Wallet app on your iPhone, Apple Watch, or other compatible device. 

Countries and regions that support Apple Pay

Countries and regions that support Apple Pay - Apple Support (EG)

Set up Apple Pay in Wallet on iPhone

Set up Apple Pay in Wallet on iPhone – Apple Support (UK)

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Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

Change, add or remove Apple ID payment methods

Change, add or remove Apple ID payment methods – Apple Support (UK)

Jaguilar510 Said:

"I would like to know why I can’t make my Apple Cash as my payment opcion"

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Troubleshooting Methods of Payment:

Use this Link: If your Payment Method is Declined in the App Store or iTunes Store - Apple Support

No Success?...

Contact Apple Support:

For Financial Assistance, contact Apple Support. These are user-to-user forums. We are not Apple.  Apple barely reads these site on their own time, as it is. As you may suspect, no one here would know the answer to this. So, contact Apple Support. The'll see what has occurred, and will run you through the fix. Here is the information for contacting Apple Support:

Apple Phone Support:

• Contact Us - Choose Locations
• Proceed from there as Necessary
• Why the Wait: Phone calls are taking a bit to go through at the moment, due do lots of calls. With the Coronavirus Pandemic, many are at home at the moment, with much time on their hands to contact Apple. So, just stay on the line, and you will get through :)