Replace the previous macro with the code below. This version forces you to enter new data in the first available blank row. It also generates the next Ref number. Because the macro is a worksheet change event, it wouldn't work to have the Master automatically open when data is entered into the "child" worksheet. However, we could open the Master automatically when the "child" workbook is opened. If that works for you, are the child files and the Master file saved in the same folder? If not, what is the full path to the folder where the Master is saved?
Dim lastRow As Long
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    If Target.Column = 1 Then
        lastRow = Columns(1).Find("*", After:=Cells(1), LookIn:=xlValues, SearchDirection:=xlPrevious).Row
    End If
End Sub

Private Sub Worksheet_Change(ByVal Target As Range)
    If Intersect(Target, Range("A:Y")) Is Nothing Then Exit Sub
    Dim fnd As Range, desWS As Worksheet
    Set desWS = Workbooks("Master.xlsx").Sheets("Sheet1")
    Application.ScreenUpdating = False
    Application.EnableEvents = False
    If Target.Column = 1 And Target.Row <> lastRow + 1 And Target <> "" Then
        Target.ClearContents
        Cells(lastRow + 1, 1).Select
        Application.ScreenUpdating = True
        MsgBox ("You have skipped a row(s)." & Chr(10) & "Please enter the 'Finding Severity' in the selected cell.")
        Application.EnableEvents = True
        Exit Sub
    ElseIf Target.Column = 1 And Target.Row = lastRow + 1 And Target <> "" Then
        Range("H" & Target.Row) = Left(Range("H" & Target.Row - 1), 8) & Mid(Range("H" & Target.Row - 1), 9, 9999) + 1
    End If
    Set fnd = desWS.Range("H:H").Find(Range("H" & Target.Row), LookIn:=xlValues, lookat:=xlWhole)
    If Not fnd Is Nothing Then
        desWS.Cells(fnd.Row, Target.Column) = Target
    Else
        desWS.Cells(desWS.Rows.Count, Target.Column).End(xlUp).Offset(1) = Target
    End If
    Application.EnableEvents = True
    Application.ScreenUpdating = True
End Sub