Excel 2013 keep crashing when i run macro

My excel 2013 keep crashing whenever i run this macro .
Is there something wrong with my code that crashes my excel .
Please help ,thank you

Private Sub Missing_UserName_Dept()

' Update User Name and Dept from Missing User name & dept xlsx file

Dim MaxRowNum, RowNum As Integer

Sheets("Simpat").Select


'Counting number of rows / Find the max number of row
    MaxRowNum = 1
        
    Do While Cells(MaxRowNum, 2) <> "" Or Cells(MaxRowNum + 1, 2) <> ""
        MaxRowNum = MaxRowNum + 1
    Loop
    

'Find the row with the USERNAME & DEPT - "NOT INDICATED" and use a vlookup Function

RowNum = 1
Do While RowNum < MaxRowNum

If UCase(Cells(RowNum, 16)) Like "*NOT INDICATED*" Or UCase(Cells(RowNum, 17)) Like "*NOT INDICATED*" Then


'Vlookup User Name from the file MISSINGUSERNAMEDEPT.xlsx
Range("P3").FormulaR1C1 = "= VLOOKUP(C[-3],MISSINGUSERNAMEDEPT.xlsx!R1:R1048576,2,0)"

'Vlookup Dept from the file MISSINGUSERNAMEDEPT.xlsx

Range("Q3").FormulaR1C1 = "= VLOOKUP(C[-4],MISSINGUSERNAMEDEPT.xlsx!R1:R1048576,3,0)"

'AutoFill formula. Copy and paste data as value
    Range("P3:Q3").Select
    Selection.AutoFill Destination:=Range("P3:Q " & MaxRowNum), Type:=xlFillDefault


'Copy and paste data as Value
    Columns("P:Q").Select
    Selection.Copy
    Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks _
        :=False, Transpose:=False
    
    Application.CutCopyMode = False


End If
Loop

    Range("P3").Select
    
End Sub