If you are the only admin of your website you can see the author dropdown box below the ..
So if you want to assign user to a post you have to make sure that user (author) exists in the users sections of your WordPress site
Go to Users > All Users
If user is not there you can add it
If you want to change the details of the author you can simply edit the user
If you are a editor of the WordPress site
then you create the new post and fill the content created by the author. Next you have to add the author’s details to the post
You can create another user and then you can see the user name (author name) and you can assign that user to the post.
How to add author to post
First you have to open the post on edit mode
Make sure you have selected the “post” tab at the right corner
Under status and availability you can select the author name
Now you can save the post
Assign author to multiple posts
Sometimes you may have to assign an author to multiple posts
You can do this by doing Bulk Edit