This error means that there is no account codes (or invalid codes) linked to the payment types you have set up in KennelBooker.

To resolve this error, update your KennelBooker Xero set up so that you have included a valid Revenue type account code / Bank type account code linked to each of your payment types

You can find the account codes and account types in Xero by going to Accounting, then Chart of Accounts.

In KennelBooker you can update your manual payment types here

https://www.kennelbooker.com/app/settings_custom.aspx#paymenttype

and you Stripe account (if using) account code here

https://www.kennelbooker.com/app/settings_xero.aspx