When I click on the VOD, a message 'This content is currently not available.' appears and it is unable to watch the VOD.

Check the 'Progress Management > View Restrict' setting in the VOD settings.

• Restriction: Instructor/Assistant/Administrator can watch the VOD, but students cannt.
• View: Students can watch the VOD.

The message appears as above because it is set to "View Restrict - Restriction', so you can change the setting to 'View Restrict - View'.