I can’t find the button to request Apple Credit card increase limit

There is no button to request a Credit Limit Increase. You use the chat feature in the Apple Card in your Wallet app on your iPhone.

Request an Apple Card credit limit increase

On your iPhone:

1. Open the Wallet app and tap Apple Card. 
2. Tap the More button .
3. Tap the Message button .
4. Type a message requesting a credit limit increase, then tap the Send button .

On your iPad:

1. Open the Settings app. 
2. Scroll down and tap Wallet & Apple Pay. 
3. Tap Apple Card, then tap the Info tab. 
4. Tap the Message button .
5. Type a message requesting a credit limit increase, then tap the Send button .

Goldman Sachs will consider your payment history with Apple Card and other credit information to determine whether to grant any request for credit limit increases on Apple Card.1 This means you may need up to six months or more of payment history with Apple Card before Goldman Sachs can approve your request for a credit line increase.

How to view your available Apple Card credit details or request a credit limit increase - Apple Support