Google API OAuth Client: Localhost never allowed for client ID

I have followed this guide, and re-visited the step multiple times, but cannot see what I am doing wrong here, if anything.

I have tried adding http://localhost and http://localhost:80, this is where my local server is running.

Whenever I attempt to prompt the user for sign-in, I am met with the error:

[GSI_LOGGER]: The given origin is not allowed for the given client ID.

I have tried to prompt using the g_id_onload div block which is described in the linked document, but I have also tried triggering the prompt via the JS API, same result. There are not many steps here, but am I missing something obvious? Does my consent screen need to be verified for some strange reason? I thought there was just a cap until it is?

Any help would be greatly appreciated!