How to create dynamic dropdown list for users who have not completed training in google sheet

Question

I have list of training assigned to multiple users. As they finish training they update status to Completed.

I need to create dropdown list that include only users who has not completed training. I am able to get list in google sheet using this [ =IFNA( FILTER($B$2:$F$2,B5:F5="")) ] function. but this function is listing users in each cell.

I have attached image for reference.

Here is the link of google sheet (https://docs.google.com/spreadsheets/d/1NkRt0vLDAqeXgbpHIUJXVyWARW5ugM86wOyxpRuvLNU/edit#gid=0)

Please advise how I can resolve this.