We checked your site and fould that your theme and some of the plugin are not updated to latest version .
Please update your wplms theme and all its plugins to latest .
- WPLMS v 1.9.9.1
- Vibe Course Module v 1.9.9.3
- Vibe CustomTypes v 1.9.9.3
- Vibe Shortcodes v 1.9.9.1
- WPLMS Dashboard v 1.9.9.1
- WPLMS Assignment v 1.9.9.1
- WPLMS Front End v 1.9.9.6
- WPLMS EventOn v 1.3.1
After update please check the unit in course and if the same issue appears then please try disabling the ajax in courses from LMs -> settings .
refer :
http://prntscr.com/98zcn1
For this you also need to enable the unit locking -> force previous unit complete from wplms -> course manager .