Ask them to double-check they are entering it into the correct place (voucher code section in checkout basket).

If it is a loyalty code, ask them when they received it? Loyalty vouchers are not valid for 4 hours until after they're generated.

Ask them to show you (or send a screenshot) of the SMS with their discount code.

Check that the voucher code has not expired.

Ask the customer to double-check the code they are entering (voucher codes are not case sensitive).

If you have multiple locations, are your marketing campaigns shared across all locations?.

Do you have any menu items excluded from voucher usage? e.g. special offers, meal deals, dish of the month etc. If you do, customers get an error message on checkout to say they cannot use a voucher code alongside a specific menu item. For example, 'This voucher cannot be used in conjunction with 'Any Large Pizza €9.99'.

We have the 'bill' button on the menu, if it is ticked, vouchers are not applicable for this item (see picture on the left).

Do you have the partially applied voucher features enabled? That means a customer will not get a 20% discount, or be able to use a loyalty code, on any menu items with this feature enabled. You should note this on the menu for your customers.