If this is a project/assignment where you have been able to preview in the past, you may have the old preview format in your .codio file
Check how you have the .codio file set up.
If you have the old settings similar to
"Box URL": "http://{{domain}}:3000/", change them to the new format
"Box URL": "http://{{domain3000}}/",
This was changed sometime ago now but when we released the new feature to protect dynamic ports to restrict previewing to the box owner (ie you) the old format can no longer be supported. See Project Settings for more on this.
If your project is using Wordpress, see WordPress Preview issues.