This post is two weeks old, but this answer might help someone else.
Here’s a work around that worked for me. Follow the instructions to add a supplemental feed in Google Merchant Center. Cell A1, the uppermost left header in your Google Sheet, should read “id” and the rows beneath should be the product IDs from “view examples” in Google Merchant Center warning (I just fixed mine and am going off of memory). The column header to the right of “id,” B1, should read “identifier_exists.” All of the rows beneath it with cells adjacent to your product IDs should read “no.” Have Google Merchant Center fetch that file, wait a few minutes and that warning should go away. Hope that helps.