Since OK Tech WS is crafted to interface Moodle with external information systems, the value of attribute idnumber is REQUIRED for entities created that way such as courses, users, cohorts ... This idnumber is likely to be the only way to later retrieve this entity since you external application does not know about Moodle internal id.
So your must provide a non empty value for course's idnumber such as its id used in your university (chem 101, CS 502...).
edit : normally the error message should have been more explicit that ws_missingvalue... something like 'required value for idnumber is missing' ; so I suspect that you did not installed OK Tech WS properly (i.e. copying the language files at the appropriate location) or if not available copy at least the english version.