You can set an organisation-wide setting for this to apply to all your courses, but you can also have course-by-course settings for more granular control, if needed.
Organisation-Wide setting:
To set the organisation-wide setting for this
Go to ACCOUNT > Organisation > Organisation details > Course settings
Find the setting "Allow students to re-enroll in courses already done"
Set or unset the option "Allow students to re-enroll in courses already done" as desired (it will be set to "on" by default initially)
Make sure you go to the bottom of the page and click "Save"
Note:
Students cannot re-enroll in a course they are already enrolled in - they must finish that course first before they will be presented with the option to enroll again (or if their enrollment has been cancelled or has expired).
Course-By-Course settings:
Organisation-wide setting:
If you wish to be able to choose this setting per-course, you need to:
turn on the "override" option at the organisation-wide settings,
and then turn on or off the setting for individual courses
Courses that you do not set will default to the current organisation-wide setting - so you don't have to set/ change all courses, just the ones that you wish to be different than your default. If you turn off this override setting at any time then any any course settings will no longer apply, unless/until you turn it back on at this organisation-wide setting.
To see which courses have an override set see/click the link under this option - it will list all the course will an override setting - you cna easily edit and change any course you wish.
Course-by-course setting:
Once you have turned on the override at the organisation-wide setting level you can then edit the "Sales and Marketing" settings of any course and find and turn on or off the "Students are allowed to re-enroll in an already completed course" setting.
Note that until you specify the setting for this course it will use the setting of the organisation-wide-setting - and a message will show to this effect below the option. As noted above you do not have to set the course setting if the organisation-wide setting is fine for this course (unless you want to make sure this course has a given setting even if the default gets changed later).
Once you have made a setting change and clicked "Save Changes" at the bottom of the page you will see a message below this setting advising that an override is now set. There is also a link there to clear the override if you want to set it back to use the organisation-wise setting instead.
Checking course settings:
You can see the result of your settings in the "Course" list screen with a new status for "Repeatable" which means students can re-enroll in this course - providing other requirements (like original enrollments must be completed, expired, or cancelled) are met.
Note: If a course is not enrollable then it is not repeatable - once it becomes enrollable then the repeatable status may change.
Wallah!