I can't set an "existing customer budget cap" to retarget within my Advantage+ Shopping Campaign

If you aren't able to set the "Existing Customer Budget Cap" inside your A+ Shopping Campaign or it is grayed out, it means you have not added your custom audiences that you want to allocate that percentage to spend to.

Here's how to add your "existing customers" to retarget and allocate a percentage to.

It's important to note that you cannot insert "engaged" or "video view" audiences. The only custom audience you can use are pixel-based or customer lists. (Email/Customers)

Select the audience you want to allocate a % to and then click the blue "Confirm" button.

Once you confirm these changes, you may need to refresh your browser with the Ads Manager in order to have the ability to set your Existing Customer Budget Cap.