Here is the response from WHMCS:
Whilst WHMCS does not have an official integration to support a direct connection to Pabbly, it does appear that Pabbly is creating their own API queries to your WHMCS installation to retrieve this data.
To resolve this error, the integration developer (Pabbly) would need to revise their logic to query GetContacts or GetClientsDetails from the data present within the GetOrders API that I assume they are retrieving order data from.
For example, using the GetOrders API will return a userid which may be used to retrieve the client details or a list of contacts for that client via another API call to GetContacts or GetClientsDetails.
The above API endpoints and our entire API index may be found below:
GetClientsDetails -
https://developers.whmcs.com/api-reference/getclientsdetails/
GetContacts -
https://developers.whmcs.com/api-reference/getcontacts/
API Index -
https://developers.whmcs.com/api/api-index/
Whilst we cannot assist with debugging or creating third-party code as part of WHMCS Technical Support, I would also recommend reaching out to our Developer Corner where our community of experts would be happy to collaborate:
https://whmcs.community/forum/41-developer-corner/
Thank you for reaching out and if you have any questions or I may be of further assistance, please do not hesitate to reply.