If you received the following notice from AdScale, it means that you're one step away from verifying your new Google Merchant Center account with Google:
Due to recent changes in Shopify's rules, we can no longer perform this step for our customers - but we are working with them to find a solution!
In the meantime, we will show you how to set up the verification code on your Shopify store.
First, open your Shopify admin page, and choose your store.
On the left-hand menu, click "Online Store", and select "Themes"
In the section with your theme, click the three dots (⋯), and then click "Edit code" in the resulting menu
In the list of files, select "theme.liquid"
Insert a new line underneath the line that contains <head>, and paste the code that you received from the AdScale system.
Press "Save", and you're all set!