I need a random number within a range, and not to duplicate the numbers in current list

jmeddaugh,

Welcome to the forum!
Why a textbox? A user can manually change its contents. I think it would be better if you used a label. So assuming you replace the textbox with a label named lblGenerate, something like this snippet should work for you:

Private Sub cmdGenerate_Click()
    
    Dim lID As Long
    
    Do
        lID = Int(Rnd() * 9999) + 10100001
        If Columns("B").Find(lID, , xlValues, xlWhole) Is Nothing Then Exit Do
    Loop
    
    Me.lblGenerate.Caption = lID
    
End Sub