When trying to register for a course that has a fixed duration, you get the error: The entered start OR end date must be within permitted range AND ON OR AFTER (NOT before) the current date.

The error message is confusingly worded, but this is not actually an IT issue.
OLR courses are "short courses" which run from those two dates (in the screenshot example, from 8/22 to 10/23). The deadline to add/drop for short courses is the next calendar day after the course starts. So add/drop for this example course concluded on 8/23. You would need to discuss this with your academic department in case they wish to file an appeal for you to have a late add.
Please contact your advisor, your academic department, or the Registrar's Office for more information.