При условии, что строки всегда имеют длину 7 символов и заканчиваются 5-значными числами
Private Sub cmdadd_Click()
On Error Resume Next 'This line skips errors... bad practice unless you really know what you are doing
On Error GoTo 0
Dim LastRow As Long
With Workbooks(REF).Sheets(REF) 'Always refer to the wb/ws, otherwise VBA will refer to the active wb/ws
.Unprotect
'for giving the serial number based on last cell value by adding plus one
LastRow = .Cells(.Rows.Count, "A").End(xlUp).Row
prfx = Left(.Cells(LastRow, "A"),2) 'Gets the prefix by getting the 2 leftmost characters of the last filled cell
nmbr = Right(.Cells(LastRow, "A"),5)+1 'Gets the last 5 characters, which is the number
'Numbers don't have leading zeroes, so if the string did they have been stripped
'This determines if that happened and if so, it adds as many leading zeroes as the number of characters is shorter than 5
If Len(nmbr) < 5 Then
For i = 1 To (5 - Len(nmbr))
nmbr = "0" & nmbr
Next i
End If
.Cells(LastRow + 1, "A").Value = prfx & nmbr
End With
End Sub