This error occurs in two cases:
CASE 1: When a product is shown on the website and app and still the error is showing.
What you can do to fix the error:
In this case, you need to re-upload the products and wait for at least 30 mins because sometimes the upload process is not completed properly.
CASE 2: When a product has been deleted from the Shopify store but still shows on GMC.
What you can do to fix the error:
You need to delete those products manually from the GMC and wait for 30 mins. Or, you may also connect with our support to help you with this.