If you're using our master navigation template, you can put this action on your final page (see image). That will disable just
the next button. Though that template should
also be disabling the next button on the last page in a lesson already, so I suspect there might be something else going on that's causing a conflict. We'd have to see your lesson to know for sure. You can start a support chat, or send us
the latest version of your lesson and we can take a look and get back to you.