Getting VBA Run time error '9' on only some machines

Thanks very much. That seemed to be the problem. The users were not on Office 365, but on Office 2017, so when I tested the original Macro on Office 2013, I could replicate the problem.

I've then changed the xlFormulas2 to xlFormulas and the macro then worked perfectly!

Apart from changing xlFormulas2, I recommend that you write the code in such a way that it does NOT reference the active sheet by default (because there is no qualifier for Range, Cells, etc.). It happens that the code will not work then.
Nor can it be optimistically assumed that the searched phrase will be found always. The day will come when she will not be found. Such a case should be programmed. I only gave a message and an exit from the procedure, but this is not always the correct approach to the problem, especially when the previous part of the macro has already made some changes to the workbook. I will not mention the lack of declarations of the variables used.
Corrected code below

Sub Check_PO()
    '
    ' Check_PO Macro
    Dim rngFound    As Range

    With Worksheets("PO_Check_Report")
        .Range("B3:B4").ClearContents
        .Rows("8:4000").ClearContents
    End With

    MasterListLastRow = Worksheets("Master_List").Range("B3").End(xlDown).Row

    With Worksheets("PO_Check_Report")
        NotFoundNextRow = .Range("A6").End(xlDown).Row + 1
        FoundDifNextRow = .Range("D6").End(xlDown).Row + 1
        FoundOKNextRow = .Range("G6").End(xlDown).Row + 1
    End With


    With Worksheets("PO_text")
        LastRow = .Range("A24000").End(xlUp).Row
        
        ' Find PO Number in text
        Set rngFound = .Cells.Find(What:="PO Number", After:=.Cells(1, 1), LookIn:=xlFormulas, _
                                   LookAt:=xlPart, SearchOrder:=xlByRows, SearchDirection:=xlNext, _
                                   MatchCase:=False, SearchFormat:=False)
        
        If Not rngFound Is Nothing Then
            With rngFound.Offset(1, 0)
                Purch_No = InStr(1, .Value, "/", vbTextCompare)
                PO_Number = Left(.Value, Purch_No - 2)
            End With
        Else
            MsgBox "Phrase 'PO Number' not found!", vbExclamation
            Stop
            Exit Sub
        End If
        
    End With


    With Sheets("PO_Check_Report")
        .Range("B3").Value = PO_Number
        .Range("B4").Value = Now()
    End With
...

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