I have tried using /learn/api/public/v1/courses/termId:99 where 99 is the term ID retrieved from the database. I also tried using the externalId switch with the term name.
Can you give the following a try:
This is because termId is a parameter for searching for courses not a course identifier.
GET /learn/api/public/v1/courses/externalId:99 would not work as the externalId references the Course Identifier that is the external course identifier (i.e. what the SIS calls the course, not the PK1 assigned by Learn) such as externalId:Math101.
Retrieving data ...