Why does Postgresql tell me the database does not exist?
You need to double quote the name. In PostgreSQL we never double quote names. If you follow the convention, PostgreSQL will convert the unquoted identifier to lower case. In your case, that which created the db used double quotes. 

Now you have a capital letter in the identifier.

From now on you have to double quote the identifier.

Not,

postgres=# DROP DATABASE RoutingApp_development;


But,

postgres=# DROP DATABASE "RoutingApp_development";