One possible cause is a payment extension that does not support Checkout Block. An extension working in classic checkout is not automatically compatible with the block-based checkout. WooCommerce documents missing payment options as a possible result. 1
Identify the changed component
Check the checkout editor to confirm whether the page uses Checkout Block or the classic shortcode. Then verify block support for the exact extension and version. The payment provider’s brand alone is not enough; different extensions can behave differently.
On staging, compare the same cart and address in both checkout implementations. If the method disappears only in the block, you have a narrower lead than “payments are broken”. If it is missing in both, investigate eligibility settings such as country and currency too.
Keep shipping inputs unchanged during the comparison, since the address and matched zone affect available delivery methods. 2
Avoid unrelated changes
Replacing an API key does not add block compatibility. Publishing two competing checkout forms is not a safe workaround either.
Use a provider-supported integration, or test a coordinated return to classic cart and checkout when appropriate. After changing anything, verify the complete sandbox purchase. A payment-method label appearing on screen is not evidence that payment processing and order confirmation both work.
Related reading
Web development and integrations
