I was approved with Apple Pay later but how do I use it in the store, if I double click the side nothing happens

Is it in a casino? Some merchants may use an MCC and be ineligible for Apple Pay Later. Merchants are categorized by MCC (Merchant Category Codes) and they’ll be ineligible for a loan. Many stores in casinos use the MCC for casinos, even though they sell a different product. Apple Pay and banks can’t overrule the MCC the casino/hotel chooses to use for credit transactions.

You can confirm this by contacting an Apple Pay Later Specialist.

Apple Pay Later - Official Apple Support

Does Apple Pay Later (APL) show in your Wallet app on your iPhone?

>> If you don't see the option to make a purchase using Apple Pay Later

You might not see the option to make a purchase using Apple Pay Later at checkout if:

• The merchant that you're shopping with doesn't support Apple Pay Later or isn't located in the U.S. 
• Your purchase contains items that are ineligible for financing.
• You're not located in the U.S., or you're in an unsupported U.S. state or territory. 
• You were approved for your first loan in Apple Wallet but didn't tap Add to Wallet before trying to make a purchase. <<

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

It should appear when you’re at the checkout of a merchant that accepts Apple Pay? What merchant were you at?