Getting Error ORA-00902 - Invalid Identifier when creating a foreign key
Your syntax is off. You need to actually define the column that has the constraint on it, and add the constraint at the end.

CREATE TABLE Vehicle
(
  Registration VARCHAR2(7) PRIMARY KEY,
  Vehicle_type_id INT,                      -- add this
  GVW INT,
  Year INT,
  Body VARCHAR2(15),
  CONSTRAINT vehicle_type_fk                -- constraint name
    FOREIGN KEY (Vehicle_type_id)           -- constrained column
    REFERENCES Vehicle_type(Vehicle_type_id)
);


Also you'll need to drop the vehicle table before the vehicle_type table, otherwise you'll get a dependency error:

ORA-02449: unique/primary keys in table referenced by foreign keys