I am using KingswaySoft adapter to migrate the data between MS CRM instances. I am using WebAPI endpoint in the connection string due to the fact that SOAP endpoint is deprecated. But I encountered an issue while writing the data into custom lookup fields and the package is failing with the below error.
An undeclared property ‘aw_accountid’ which only has property annotations in the payload but no property value was found in the payload. In OData, only declared navigation properties and declared named streams can be represented as properties without values.
Reason:
KingswaySoft adapter making WebAPI calls to create records in MS CRM since I selected WepAPI endpoint in my connection string. But with WebAPI calls, we have to pass entity schema name also to migrate the lookup data and KingswaySoft adapter has no option to pass this data.
Solution:
Unfortunately, there is no solution at the moment. I contacted KingswaySoft support team and this is the response I got it from them.
As you have already seen, the issue that you have experienced is a limitation on WebAPI, and from the cases we have seen it mainly happens to custom fields. This limitation is documented in our software. We recognize this is a blocking issue, so we recommend to not use WebAPI for the time being until the issue is addressed by Microsoft – we did highlight a number of other issues with WebAPI in our software tooltip in the connection manager. As of now, SOAP is still available and is highly reliable than WebAPI.
So, I solved this issue by changing the endpoint to SOAP in the connection string. Hope Microsoft is working on this issue and will solve asap.
Phani Rajasekhar 
This post was very helpful, thanks for sharing
See this article for a resolution: https://carldesouza.com/the-trick-to-updating-custom-lookups-using-the-dynamics-365-web-api/
Thank you so much. This article has been really helpful and the SOAP connection manager trick worked for most of the part but while trying to migrate email attachments, if I use the WebAPI connection manager then I get the same error and when I use SOAP then destination component just keep processing the data and never ends. (No timeout error, not exceptions and no records created) Just keep loading for hours
It may be a problem with Batch Size. Try to reduce the batch size and execute. I even tried with 1 in the batch size in some scenarios to make it work.
Thanks for the response Phani. I did try that as well by following some of the instructions from this link:
https://community.dynamics.com/crm/f/microsoft-dynamics-crm-forum/260092/migrating-email-attachment-using-kingswaysoft-ssis/730649
but it did not work. For now I created a console application and migrated the attachments successfully. It is fine for me until I find a better solution.