EXCEL VBA: For Loop involving checking Duplicates and continuing serial - excel

Question

I am new at using VBA and I am trying to do something that seems "simple." I have my VBA code generate a string (CP20210100001) and I want my for loop to check if that string has already been used in that column. If already used, generate the next in the serial until the next unique value in the serial has been generated.
My boss wants to paste a different ID occasionally in the column and this disturbs the code. My code looks at the last row and adds one to the String + serial. This will result in duplicates.
I figured out through much googling to get the code to check the current value for duplicates but I can't figure out how to get it to check for future IDs in the series until it comes across a unique value.
Below you can see my column. I had 10 successful submission and then my boss pasted 3 rows. With my VBA the next generated ID would be CP20210200004 but last part of the code found it as duplicate so it added 1 and inputted CP20210200005. Ideally the VBA should for loop until the next in the serial shows up. In this case CP20210200011. This way no matter how many times my boss disrupts my table, my ID sequence stays in tact.
**Reference ID**
CP20210100000
CP20210200001
CP20210200002
CP20210200003
CP20210200004
CP20210200005
CP20210200006
CP20210200007
CP20210200008
CP20210200009
CP20210200010
JS20210200001
JS20210200002
JS20210200003
CP20210200005
Below is the the VBA
#Timestamp is part of the String + Serial Combo
Timestamp = Format(Year(Date)) + Format(Month(Date), "00")
#I found this online. Essentially if A2 is blank then input CP + Timestamp + 00001 (CP20210100001)
#It looks at the last row to find the old value (OVAL) and generate the new value (NVAL)
If Sheets(ws_output).Range("A2") = "" Then
Sheets(ws_output).Range("A2").Value = "CP" & Timestamp + 1
Else
lstrow = Sheets(ws_output).Cells(Rows.Count, "A").End(xlUp).Row
Oval = Sheets(ws_output).Range("A" & lstrow)
NVAL = "CP" & Timestamp & Format(Right(Oval, 4) + 1, "00000")
#Here I am trying to see if NVAL is a duplicate value. If so add one to the serial.
Count = Application.WorksheetFunction.Countif(Sheets(ws_output).Range("A2:A100000"), NVAL)
Dim Cell As Range
For Each Cell In Sheets(ws_output).Range("A2:A100000")
If Count > 1 Then
NXVAL = NVAL
Else
NXVAL = "CP" & Timestamp & Format(Right(NVAL, 4) + 1, "00000")
End If
Next
Please please please help.
EDIT
I Should clarify that all of this is triggered on a form. The module is connected to a submit button. Once the button is pressed all the values in the form write to a separate sheet. Reference ID is the only part that isn't on the form. Essentially once the button is pressed, it triggers the query to write the next available reference ID. The next line in the query is
Sheets("Sheet2").Cells(next_row, 1).Value = NXVAL
I need the new Reference ID to equal a variable.

Answer 1

Your code seems to give you much grief and little comfort. The reason is that you didn't take a strictly logical approach. The tasks are ...
Find the last used number. I suggest to use VBA's own Find function.
Insert the next number. It consists of prefix, Date and serial number.
So, you arrive at code like this:-
Sub STO_66112119()
' 168
Const NumClm As Long = 1 ' 1 = column A
Dim Prefix As String
Dim LastNumber As Long
Dim Fnd As Range ' search result
Prefix = "JS" ' you could get this from an InputBox to
' enable numbering for other prefixes
With Columns(NumClm)
On Error Resume Next ' if column A is blank
Set Fnd = .Find(What:=Prefix, _
After:=.Cells(1, 1), _
LookIn:=xlValues, _
Lookat:=xlPart, _
SearchOrder:=xlByRows, _
SearchDirection:=xlPrevious, _
MatchCase:=False)
End With
LastNumber = Val(Right(Fnd.Value, 5))
On Error GoTo 0
Cells(Rows.Count, NumClm).End(xlUp).Offset(1).Value = Prefix & Format(Date, "yyyymm") _
& Format(LastNumber + 1, "00000")
End Sub
You need to spend a moment on preparation, however.
Define the column to work in. I put this in the Const NumClm. It's at the top of the code so as to make maintenance easier (won't need to dig in the code to make a change).
My code shows Prefix = "JS". You want to change this to "CP". I inserted "JS" to show that you could use any prefix.
The above code will continue counting up in a new month and even a new year. If you want to start each year with a new series just change the way you handle the found previous. The Find function will return the cell where the prefix was last used. You might further examine that cell's value.