Missed double quotation for ""Check"" in the formula and need to add a bit of trick.
Sub test()
Dim r As Range, ff As String, flg As Boolean
With Range("C4:AZ10000")
Set r = .Find("Check", , xlValues, 2)
If Not r Is Nothing Then
Do
r.FormulaArray = "=IFERROR(INDEX('Leave'!$G$1:$G$10432,MATCH(1,IF('Leave'!$A$1:$A$10432=$A8,IF(AC$1>=" & _
"'Leave'!$I$1:$I$10432,IF(AC$1<='Leave'!$I$1:$I$10432,1))),0)),""Check"")"
If (Not flg) * (r.Value = "Check") Then ff = r.Address: flg = True
Set r = .FindNext(r)
If r Is Nothing Then Exit Do
Loop Until ff = r.Address
End If
End With
End Sub
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