If you are receiving payments via American Express then you will need to ensure you have this Payment Method in your Merchant Account.
This is normally configured during your KYC process with the SmartHOTEL Team, however it may be set up at a later stage.
SmartHOTEL requires the MID number from AMEX (9 digits) to get this payment type set up. You can register as a merchant via this link: https://www.americanexpress.com/uk/merchant/accept-the-card.html?inav=gb_menu_business_merch_acceptcrd. Once you have your MID please provide this to SmartHOTEL.
When setting up your MID with AMEX please ensure that the correct Merchant Category Code is being used. You will need 7011 which relates to LODGING, HOTELS, MOTELS, RESORTS
If you do not have the AMEX payment method set up, you may see the following Aquirer Response