The issue here is with the "Number Of Promotion Places" field in the second screenshot.
This field refers to the number of teams automatically promoted as a result of the attached stage (in this case, the two groups of 20 teams), resulting in 6 teams promoted from the group tables.
With the additional 1 team from the playoffs this results in 7 teams total being set for promotion, for which changing the "Number Of Promotion Places" field to 1 will resolve the error.