WordPress post author dropdown box not showing?

For Newly created users:

1. Open the wordpress admin dashboard.
2. Next go to users -> add new.
3. 
4. create user by filling necessary details.
5. 
6. when filling user role choose user role as author. other users will not have permission to create or edit blogs.
7. 
8. You can check now in posts now. That drop down will be visible now.
9. 

For Already Created users:

1. For already created users please check their role. if it is subscriber or customers it will not come in drop down.
2. Why because those roles don’t have a privileges to edit or create the post or page.
3. You can use the following roles author, editor, administrator who are all having a permissions to edit the posts or pages.