This problem affects only schools that use a site license for payment instead of access codes.
This is a known issue that will be fixed in an upcoming release.
Resolution
Refresh the page in your browser and start again.
Can't Join Spark Course After Entering Incorrect Course Key
When joining a new Spark course, if you enter an incorrect course key, you cannot join the course after entering the correct course key.
SparkPrimary StudentCourse Setup