In your expression you are using Max() which is require Group By statements and grouping on the dimension, so you can't use with out using Group By clause in your statement.
if(match(Timesheet_Weekno,Max(Timesheet_Weekno)),[Project Manager Name]) as MaxWeekProjectManagerName
[Project Manager Name],
Please read about the Aggr or Group By.
You can left join the MaxTimesheet_Weekno field while getting Max Timesheet_Weekno and then use this field for calcualtion.
Max(Timesheet_Weekno) a MaxTimesheet_Weekno
if(match(Timesheet_Weekno,MaxTimesheet_Weekno),[Project Manager Name]) as MaxWeekProjectManagerName
Drop Table Source;