SESSION: ci_sessions database table not populating

Is there a Session.php file in the root of your /system/libraries/ directory? If there is, then you have missed the step of deleting the contents of the directory before copying over the files from CI3.

If there is not a Session.php file in the root of your /system/libraries/ directory, then your error message should have at least changed. If it did not, then make sure you don't have a Session.php file in your /application/libraries/ directory. Of course, this would imply that you were using some sort of customized Session library before and you may still have other issues to resolve in migrating to CI3.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide