Assistance with Pivot table mapping

That formula for the 1,1 box looks fine.  I might recommend using D$5 and $C4 instead of the 1 and 1 so you can just copy and paste the formula into the other 8 squares.

As for the problem assuming you have emplyees rated with 1,1 then check to make sure the rating entries are numbers or in the formula force convert the ratings to numbers or text.  for example:

make the 1,2,3 in the grid TEXT (e.g. pre-format cells as text, or use '1 or ="1") 

FILTER(TalentTable[Employee],(""&TalentTable[Performance]=D$5)*(""&TalentTable[Potential]=$C4),"")

or 

FILTER(TalentTable[Employee],(--TalentTable[Performance]=D$5)*(--TalentTable[Potential]=$C4),"")

but in this case if you have something else in the performance or potential columns that will cause an error