Every blog can be written by authors who are marketers, contributors, or content specialists. Authors should be added first in the blog builder (as shown in this article) before they can be selected as the authors for the blog post.
How to add authors to the blog builder?
Click on "Sites" > Blogs > Settings (cog)
Click on "Author" tab located on the left side > "+ Add new author"
Add your author name, photo, image alt text, social pages, and author description > Click on "+ Create Author" once you are ready
Note - Supported Image size for Author is 10MB