The problem: your Adyen web drop-in is embedded in an iframe. The Apple Pay button renders fine, but tapping it opens the sheet and instantly closes it with "Payment not completed". The drop-in validates the Apple Pay merchant session against the iframe URL, which Apple refuses, so the sheet can never complete.
What to do:
1. Upgrade @adyen/adyen-web to 6.23.0 or newer. The SDK added support for this exact case in PR #3561, changing how the merchant session validation URL is picked.
2. Configure the Apple Pay component explicitly in paymentMethodsConfiguration, as shown in the thread, instead of relying on the drop-in defaults.
3. Retest the iframe flow. Reporters confirmed the simpler config on 6.23.0+ stops the instant dismissal and payments go through.
Dont waste time debugging your merchant certificate or domain registration for this one: if Apple Pay works on the top-level page but dies inside an iframe, its the validation-URL bug this release fixed.