Hi Carlos - Where are you trying to open the "activity completion" option? Please share the steps or a screenshot. Do you have "completion tracking" enabled at the site level? And is "course completion form" a class that you have added? (If so try to purge all caches.)
Hi Carlos,
In addition to the comments made by Ravi and Florence, can you indicate what activity module you're trying to setup/edit? Sounds a bit like a 3rd party plugin, judging by the error message you got. Anyway, a screenshot of the error on the activity edit settings page (or add activity for that matter) would greatly help in determining where to solve the issue.