You may encounter this message if you integrate your Bookeo account with MailChimp.
The full message would read something like this:
"Your MailChimp list contains some custom fields that Bookeo knows nothing about.
Bookeo cannot subscribe customers to your list if these fields are required.
Please remove these custom fields from the list, or mark them as not required.
Original error: XYZ must be provided - Please enter a value"
This means that in the settings for your MailChimp list, you have set up a custom field as required, but Bookeo does not know about that field, and so MailChimp refuses to accept the subscription.
Bookeo only sends FNAME (first name), LNAME (last name) and obviously EMAIL. These are the only default fields in MailChimp.
Make sure to not mark as required other fields in the list, or use a different list for Bookeo.
Please see below a screenshot showing the settings page where the XYZ is set as required in this example.