Hi Gary, the mark complete/incomplete button is still available. It can be located in the progress section. The button does work properly, I think where the confusion lies is that there is no way to mark incomplete once the lesson is 100% complete. You can find full details about this here: https://www.guitartricks.com/blog/how-to-track-your-progress-on-guitar-tricks
Let us know if you have any further questions.
I have to respectfully disagree. It is not working. If a lesson is showing 3 out of 6 complete in the progress section, there is no incomplete button. Only a Mark Complete button. If you hit the Mark Complete button it changes to 6 out of 6 complete and the button changes to say "Mark Incomplete". If you then hit the Mark Incomplete button it goes back to 3 out of 6 complete. Right back to where you started.
edited
Gary
[Epiphone Les Paul Standard Plus Top Pro (2), Fender Player Stratocaster (2), Fender Player Telecaster (2), Squire CV 60's Stratocaster, Hamer Ecotone, Yamaha APX600 (2), Epiphone ES-339, GTX-100 (2), Spark 40 (2), Spark Mini.]