Error - Unable to resolve the request
As I thought, I was just being silly. I initially didn't realise that custom controllers need to be encapsulated within a custom plugin

I guess my mistake was going directly to the controllers documentation and missing the line that says:


  To create a controller, first create a new controllers/ subfolder within your plugin’s folder


Hopefully anyone who runs into this (doubtful as I can imagine most, unlike me, will probably read the documentation properly) will find it somewhat useful