Please check the following points :
1. Note that review form only shows to the users which are subscribed to course(taken that course) .
2. Check if the comments are enabled from back end edit course page .
refer : http://prntscr.com/avqmov
3. Check if you have created any page named course and set it as all courses page (course directory) in settigs -> buddypress -> pages -> course .
IF yes then please delete that page named ” course ” in your site as it will create conflicts in permalinks structure of wplms .
We strictly do not recommends to create such page ,post,quiz ,course etc named as course which will have slug as course .