OAuthException: redirect_uri isn't an absolute URI. Check RFC 3986 (Symfony)
I don't know where you got the example code, but certainly not from the linked tutorial.
Facebook authorization is based on the fact that you generate a link to FB, the user goes to the FB and authorizes himself, and then the FB server redirects it back to you (along with whether or not it is authorized).
FB does not guess where to redirect user after login. You need to give him a full path with http(s) and the server name (and if I remember correctly, it is also compatible with that saved in the FB app)
The attached tutorial requires writing a controller with 2 methods (output and return) and corresponding entries in the configuration.
If you use this, then see how you have configured the provider. What is in redirectUri?