Complete button not working

Hi!

The complete button of the lesson is not working at all. I don’t know is it related to this but also today I have updated the plugin. Unfortunately I’m not sure was it broken before the update. Although I have rolled back the previous version and still is the problem present.

The site is not yet live and I was making some adjustments and installed a few plugins, but after discovering the problem I disable them, but the problem persists. As far as I can tell there is no other modification.

Can anyone point me to the solution or some way to debug it?

Many Thanks!