It has been over a year and I haven't found any solution to this. I've repeated this process every quarter or so, hoping that Google has updated their system to support shopify local feed but I still get the same rejection notice.
The only solution I can think of is to specify SKU for your products and use the SKU instead of item id as the id for your local feed. Alternatively you can purchase the app to sync it automatically for you.
From what I can understand, by changing the SKU as id, it will refresh and reload the whole google ad process again so I do not wish to take the risk.
Hope you can find a solution and do share them when you can
Understood, please note it takes me nearly 1 to 2 months to complete this task. I have done it several times, I get roughly 1 or 2 clients per month that need help setting it up. And it just takes a long time, even for someone who knows what they are doing