Need Help! KPI Visual (Indicator / Trend / Target) issue

You'll want to put Week No in the trend axis. You may also want to start this as a matrix visual and make sure it is sorted correctly before turning into KPI visual: 

https://docs.microsoft.com/en-us/power-bi/visuals/power-bi-visualization-kpi

The reason you're getting blank for the Goal is because your current measure has too many filters: 

Previous Week Total Workforce = CALCULATE(SUM(Workforce[Total Staff]), FILTER( ALLSELECTED('Workforce'), 'Workforce'[Week No] = MAX(Workforce'[Week No]) - 1 ) )

The ALLSELECTED('Workforce') is filtering the workforce table to only show the currently selected Week No.

Then 'Workforce'[Week No] = MAX(Workforce'[Week No]) - 1 is filtering to show the previous Week No. 

There are no records that exist in both the selected Week No AND the previous Week No, so your goal shows blank. 

You need to either use a filter that SHIFTS the current filter context (like DATEADD or Datesbetween) or try adding an ALL(Workforce[Week No]) into the Goal somewhere.