If you drop grades during the semester, you will need to semi-manually change all incomplete assignments to zero before you can finalize grades.
There is an easy way to do this in the grade book. Click on the pull-down arrow at the top of an assignment. Click on 'Set Default Grade'. Enter 0 as the grade, and make sure you do not overwrite grades that are already entered. Canvas will automatically change all "-"s to "0"s and leave the graded assignments alone.