I get the subject error message with this VBA code. If I type the formula directly in the cell, there is no issue.
Sub index_multiple()
ActiveCell.FormulaArray = _
"=INDEX('[zCurrent Rates (QUERY).xlsx]Rates'!$C:$C,MAX(('[zCurrent Rates (QUERY).xlsx]Rates'!$Q:$Q=C2)*ROW('[zCurrent Rates (QUERY).xlsx]Rates'!$Q:$Q)))"
End Sub
Sub index_multiple()
ActiveCell.FormulaArray = _
"=INDEX('[zCurrent Rates (QUERY).xlsx]Rates'!$C:$C,MAX(('[zCurrent Rates (QUERY).xlsx]Rates'!$Q:$Q=C2)*ROW('[zCurrent Rates (QUERY).xlsx]Rates'!$Q:$Q)))"
End Sub