You could reduce the formula to:
Code:
=IFERROR(INDEX(Details!B:B,IF(SUMPRODUCT(--(Details!A:A=B2),--(Details!C:C<=D2),--(IF(Details!D:D<>"",Details!D:D,TODAY())>=D2),ROW(Details!A:A)-ROW(Details!A2)+1)=0,"",SUMPRODUCT(--(Details!A:A=B2),--(Details!C:C<=D2),--(IF(Details!D:D<>"",Details!D:D,TODAY())>=D2),ROW(Details!A:A)-ROW(Details!A2)+1))),"NO TENANT PRESENT")
As for the 'NO TENANT PRESENT' result throughout, that's because your Details sheet has strings, not dates, for the 'CUSTOMER START DATE' & 'CUSTOMER TERMINATION DATE' columns.