The short answer
Google Customer Reviews — the Merchant Center survey programme at customerreviews.google.com — is not a supported source and cannot be collected automatically.
Google Business Profile reviews, which is what most people mean, are fully supported.
Which one you have
If your reviews came from a post-purchase survey Google emailed your buyers, and they feed a seller rating in Shopping ads, that is Google Customer Reviews. If they are the reviews on your Maps listing, that is a Business Profile and it works.
The full difference is explained here — it is a 6,600-a-month confusion and worth two minutes.
The workaround
Add them by hand, one at a time, in the All-in-One app. Tedious for a large set, fine for a handful of strong ones.
If you want it supported
It is on the wishlist and votes are counted.