I am getting this error when I have upgraded my CakePHP to 3.6.6 and added new webroot/index.php which is calling Cake/Http/Server and App/Application classes.
Have you created the newly required Application class? If yes, can you paste it here? If you have created it, you are probably missing a use statement.
This got solved, was a namespace issue as you suggested.
I have got a new error - [InvalidArgumentException] The “cake_routes” cache configuration does not exist.
Any idea about this?
I just run into the same a few hours earlier. In my case my CakePHP version and the Application.php file’s version was not the same as it seems to be non upgradeable by cake’s upgrade process. Replace your Application.php and out back your own customization.