For each lesson type, ensure the completion settings are robust as possible. You may also consider marking certain lessons as prerequisites under Lesson Settings. This helps ensure students are working through content in the correct order and uses some criteria for earning coins.
Video: Go to Course Settings / Course progress & completions / set the video completion to 100%
Quiz: Go to Lesson Settings / require a passing grade. To increase the difficulty/threshold to earn coins, consider increasing the passing grade percentage.
Assignment: completions are based on the Instructor review. The status needs to be Approved in order for coins to be awarded.
Text, Survey, Pdf, Audio, Presentation, Download, Multimedia: these lesson type completions are manually marked by the student.