This root cause usually comes down to these two things:
Inside of AdWords go to your Audience Sources > Audiences > AdWords > and then click into the Item ID error like this:
Once here, check to see if you have the Custom ID parameter checked and then you'll see the Item ID is actually the dynx_itemid:
Since you're a retail store, we are using the proper parameters - Retail - and sending these via ecomm_prodid shown just above the custom parameters.
If you have the custom parameter setup and aren't sure why, it could come from a number of sources such as previous apps, developers, agencies, team members etc.
While you can't remove this setting yourself, the good news is that this false positive message will go away on its own.
From our experience, most of the time this is a false positive if you've set up our app in its entirety and you do not have products on every page of your site that require this to be 100% of pageviews are sending ecomm_prodid's.
According to Google's own requirements found here only PRODUCT and CART pages should send the ecomm_prodid in the remarketing tags:
Which is exactly how our tags are created.
And since you're an eComm site then you have many other pages that users view (home, collection, search etc) which would take up a large % of event hits.
So as long as you see some ecomm_prodid data in the graphs below this main warning message like this, then you are likely OK.
If you have any questions please don't hesitate to contact us!