Okay, the challenge you have there is your completion column is User Specific, which means that each user can only see their own status for each task.
If you want others to see it, you need to take a different approach. One approach would be to instead use a non-user specific Multiple Files column to store an array of UserIDs, and add each users ID to that column as they complete each task.
The below tutorial walks you through how to set that up: