“Restriction is applied to your order…”, when Coupon applied

On my Magento 2.2.7 instance, when a coupon is applied in the checkout, the error message “Restriction is applied to your order. Please review shipping details on Shipping step.” is displayed for 5 secs and then disappears. It doesn’t stop the order being placed.

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What does this error mean and how do I stop it being displayed?

Cheers Rob