Why are my website’s Accelerated Mobile Pages are not indexed in Google?

Check if the URLs are in the Google AMP cache, see https://developers.google.com/amp/cache/overview#amp-cache-url-format.

It does not mean necessarily that the URLs will appear in search results in the AMP carousel, but it is an indication that they are considered as valid AMP pages by Google.

Check also the structured data mark-up, see: https://developers.google.com/search/docs/guides/mark-up-content#use-amp-html.

AMP pages appear in the top stories carousel depending on what search query you use, and on the country. You might not see them in search results, but other people might, and vice-versa.