Like this

=IF(MIN(IF('Perform Billing Audit Report'!$B$2:$B$1500=Details!$D7,'Perform Billing Audit Report'!$E$2:$E$1500))=--"2020/01/01",
"No Start Date",
MIN(IF('Perform Billing Audit Report'!$B$2:$B$1500=Details!$D7,'Perform Billing Audit Report'!$E$2:$E$1500)))

array entered.