When a course is already In Progress, you can’t unassign it directly.

Use a Learning task to create a new enrollment and reset the learner’s progress, then unassign the original enrollment.
To reset the learner’s progress and remove the original enrollment:
- Go to Tasks > Task Lists Settings.
- Click + Create task list.
- Add a title, description, and folder for the task list.
- Choose a task list owner.
- Set the time of day and time zone for the task delivery.
- In the Task List builder, click + > Create task.
- Select Learning task.
- Choose the relevant Learning Path or Course.
- You’ll need to type at least one word from the course name in the search field for it to appear.
- Save the task list.

Once the task list is ready, manually trigger it for the employee through his profile in Bob.
This will create a new enrollment for the course, which resets the learner's progress. Once that new enrollment is created, you should then be able to go back to People's Learning, locate the employee, click on Actions, and select Unassign Learning to remove the original in-progress enrollment.
This workaround is needed because Bob currently doesn’t allow a course to be directly unassigned while it is In Progress.