A customer enters a Coupon Code in the cart and receives an error stating that...

According to the Adobe Commerce Merchant Documentation, there are several reasons why a coupon code might be invalid, such as expiration date, coupon code format, or coupon usage limit3. Two of these reasons are related to customer group and uses per customer settings in the cart price rule. If the customer belongs to a customer group that is not included in the cart price rule, or if the customer has already used the coupon code more than the allowed number of times, they will not be able to apply the coupon code.

References: 3 https://docs.magento.com/user-guide/marketing/price-rules-cart-coupon.html#troubleshooting