Remove the last comma, or add null
Case([Priority],
1, WeekDayAdd(ToDate([Date Created]), 1),
2, WeekDayAdd(ToDate([Date Created]), 2),
3, WeekDayAdd(ToDate([Date Created]), 3),
4, WeekDayAdd(ToDate([Date Created]), 4),
5, WeekDayAdd(ToDate([Date Created]), 4),
6, WeekDayAdd(ToDate([Date Created]), 5),
7, WeekDayAdd(ToDate([Date Created]), 6),
8, WeekDayAdd(ToDate([Date Created]), 7),
9, WeekDayAdd(ToDate([Date Created]), 8),
10, WeekDayAdd(ToDate([Date Created]), 10)
)
OR
Case([Priority],
1, WeekDayAdd(ToDate([Date Created]), 1),
2, WeekDayAdd(ToDate([Date Created]), 2),
3, WeekDayAdd(ToDate([Date Created]), 3),
4, WeekDayAdd(ToDate([Date Created]), 4),
5, WeekDayAdd(ToDate([Date Created]), 4),
6, WeekDayAdd(ToDate([Date Created]), 5),
7, WeekDayAdd(ToDate([Date Created]), 6),
8, WeekDayAdd(ToDate([Date Created]), 7),
9, WeekDayAdd(ToDate([Date Created]), 8),
10, WeekDayAdd(ToDate([Date Created]), 10),
null)