What does an AggregateRating contain?
Three facts: what was rated, the average, and how many ratings or reviews it's based on. In schema.org terms that's `itemReviewed`, `ratingValue`, and `ratingCount` or `reviewCount`. Google requires at least one of the two counts, and numbers default to a five-point scale unless the markup sets `bestRating` and `worstRating`.
Google adds two conditions on the page itself. "If you use AggregateRating, users should be able to see that aggregate rating on the page," and for local businesses, "Ratings must be sourced directly from users," not compiled by editors. So the markup has to describe a visible rating that real customers gave, never a number typed into the code.