Exception - Class not found error while trying to add a function to external service

Hi,

I have added Custom service to my moodle but when i click on Add function to this service it gives me an error:

Exception - Class 'mod_artsslesson\external\artsslesson_summary_exporter' not found

Full error:

My moodle version - Moodle 3.9.3

Can anyone clarify whats this error is about?