Why am I am getting an alert in my Google Merchant Center for this account saying I need to set-up a product listing ad when I already have one set up?

Thank you for your email.  For the alert, it might be a general notice that Google posted for all accounts and may not be specifically targeting your account.  You can contact Google through the contact form in your Google Merchant account to make sure they have marked your account appropriately.  In cases where merchants haven't established their product listing ads (PLA) campaign, Google delists all items and merchants see an error message that 0 items are inserted.  My guess is the message you see is a general warning reminding merchants they need to establish the PLA campaign.