Sorry for the inconvenience. I'll check with the NSE team if there is an issue with the enrollment and get back to you soon.
One thing that can cause this error is if you use old "deep links" into the courses - usually by bookmarking individual courses. Try navigating to https://training.fortinet.com/ and use page navigation to get to the NSE 2 and NSE 3 courses.
Also, for the record, the sales training courses in NSE 2 and NSE 3 are only available to Fortinet employees and channel partners.
NSE 2 and 3 courses are not open for public enrollment. Are restricted to Employees and Partners only.