Two things which I was able to notice in the screenshots you shared
- Your date field is "Date" with a Capital "D". But, your expressions have "d"
- The date format on Date Field is DD/MM/YYYY, I hope your environmental variable is also set to same
SET DateFormat='D/M/YYYY';
Anyway, try
=Sum({$<[Full Name]={'Darren Keirl','Chris Phillips','Gemma Young','Micheal
Heaney','Ginnie Brunning','Terry Crampton','Lisa Lewer','Emma
Hares','Julita Wakefield','Liz Halliwell','Sandra Searle'},
Date={">=$(=Date(Date#(Max(Date), 'DD/MM/YYYY')-14), 'DD/MM/YYYY')<=$(=Date( Date#(Max(Date), 'DD/MM/YYYY'),'DD/MM/YYYY'))"} >}
[TotalExternalInboundAnswered]+[TotalExternalInboundAnswered])
.png "Former Employee"){kind=icon}
