writing a new service - Class \"external_function_parameters\" not found

Hi everyone.

I used the exact same instructions in moodle documantaions to create a web service. Yet when I use postman to send a request to the relevant endpoint, I recieve an error with the message: "Class \"external_function_parameters\" not found". Furthermore when I go to 'site adminstration->developement->webservice test client' I get the same error.

I tried "require_once('lib/externallib.php');" as suggested in functions doc page upon which the error message changed to "Failed opening required 'lib/externallib.php' (include_path='[dirroot]/lib/pear;C:\Moodle_403\server\php\PEAR')".