Runtime error 13

Because here the Target is not a single cell. It's a range of cells.

So, change your current code like this...

Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)
    If Sh.Name <> "Master" Then
        If Intersect(Target, Range("B:B")) Is Nothing Then Exit Sub
        If Sh.Cells(Target.Row, "B").Value = "x" Then
            Sheets("Master").Range("ZZ1") = Sheets("Master").Range("ZZ1") + 1
            Target.Offset(0, -1) = Sheets("Master").Range("ZZ1")
            Target.EntireRow.Copy Sheets("Master").Cells(Sheets("Master").Rows.Count, "A").End(xlUp).Offset(1, 0)
        ElseIf Sh.Cells(Target.Row, "B").Value = "y" And Sh.Cells(Target.Row, "A").Value <> "" Then
            Sh.Cells(Target.Row, "A").Value = Sh.Cells(Target.Row - 1, "A").Value + 0.1
            Sh.Cells(Target.Row, "B").EntireRow.Copy Sheets("Master").Cells(Sheets("Master").Rows.Count, "A").End(xlUp).Offset(1, 0)
        End If
    End If
End Sub

Also use with statement for sh to avoid calling it many times (like the below)

Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)
    
With Sh
    If .Name <> "Master" Then
        If Intersect(Target, Range("B:B")) Is Nothing Then Exit Sub
        If .Cells(Target.Row, "B").Value = "x" Then
            Sheets("Master").Range("ZZ1") = Sheets("Master").Range("ZZ1") + 1
            Target.Offset(0, -1) = Sheets("Master").Range("ZZ1")
            Target.EntireRow.Copy Sheets("Master").Cells(Sheets("Master").Rows.Count, "A").End(xlUp).Offset(1, 0)
        ElseIf Sh.Cells(Target.Row, "B").Value = "y" And Sh.Cells(Target.Row, "A").Value <> "" Then
            .Cells(Target.Row, "A").Value = .Cells(Target.Row - 1, "A").Value + 0.1
            .Cells(Target.Row, "B").EntireRow.Copy Sheets("Master").Cells(Sheets("Master").Rows.Count, "A").End(xlUp).Offset(1, 0)
        End If
    End If
End With

End Sub