I am attempting to set up the authentication for a Google login in my Azure App Service.
However In my App Service the 'Authentication/Authorization' section is not available.
What do I need to do to enable this???
From your description it seems you are based on Linux Consumption plan (because only this host plan doesn't support authentication/authorization feature).
The solution is give your function app another host plan, such as Linux Premium plan.
This is the official document:
https://learn.microsoft.com/en-us/azure/app-service/configure-authentication-provider-aad
If you use Linux Consumption plan, then any way of authentication/authorization is impossible.