Upgraded to wordpress 5.7 and now I can’t select myself as a post author

I’m not sure if the issue you are facing is relating to your role?

If your role is set as Author, then by default you will be the selected author of the post and you are not able to select the author on creating a new post. Checking the WordPress Codex under ‘Roles and Capabilities’ shows that only roles with the capability of edit_others_posts can change the Author of the post.

You can find this link to a table showing the roles and their related capabilities.

If this is not the issue you are having, please deactivate your plugins and set the theme to a default theme, such as ‘Twenty Twenty One’ and see if you are experiencing the same issue.