Error on Updating Existing Stock Item to be a Matrix Item

Hello,

We are in the midst of implementing the Commerce Connector from Acumatica(build 21.208.0032) to an existing BigCommerce store. We are implementing in phases, with the first phase being stock and non-stock items, and inventory levels. We have a custom built integration that imports orders from BigCommerce. Most of our products need to be Matrix items with only 1 variant. I have followed the steps outlined here:

With the exception of importing the items as we already have the items created in Acumatica. When trying to update a stock item to be tied to a template item, I get the following error:

I have been able to use that attribute to create new matrix items following our part number format. I would greatly appreciate any help in resolving this error. Let me know if any further info is needed.

Thanks!