Why doesn't my warning message show when the customer pays? (Essential Checkout app)

A validation rule has two types:

• Block checkout: the customer can't pay until they fix the issue.
• Warning message: the message shows while the customer fills in the checkout, but it doesn't stop them from paying. Warnings are not shown when the customer completes the checkout.

If the message must appear when the customer clicks to pay, choose Block checkout and tick Completing the checkout under Show message when.

Also check Position. If the message is attached to a specific field, it shows next to that field and is easy to miss. While testing, choose Top of the checkout page. See Validate and Block Checkout on Essential Checkout.