Inputbox do until two letters are entered

Question

This is my first time working with an InputBox. The desire is to have the user insert their initials for entry into a spreadsheet that will be imported into a database. I'm using the InputBox to promote consistency and AutoFill the necessary cells.

I am having trouble understanding a process whereby a user enters information, if the entry is two letters its accepted and populated into the cells, else a message appears indicating two letters are required and the InputBox displays again. Through testing I believe my loop is not working as I would expect. If the first entry is two letters it populates information into excel as expected. If, however, the first entry is incorrect and a subsequent entry is correct it does not seem to exit the loop. I'm not sure why this would be? Any help would be greatly appreciated.

Dim c As Range

Set c = Sheets("CompilePriceAdjustments").Range("E2")

    c = InputBox("Please Enter Initials", "PRICE INCREASE APPROVER")
Do Until c = vbString And Len(c) = 2
    MsgBox ("You must enter two letters")
    c = InputBox("Please Enter Initials", "PRICE INCREASE APPROVER")
Loop

Sheets("CompilePriceAdjustments").Range("E2").Value = UCase(c)
c.AutoFill Destination:=Sheets("CompilePriceAdjustments").Range("E2:E" & Cells    (Rows.Count, "D").End(xlUp).Row)

Answer 1

I think this is what you are trying?

Sub Sample()
    Dim c As Range
    Dim Ret

    Set c = Sheets("CompilePriceAdjustments").Range("E2")

    Ret = InputBox("Please Enter Initials - (Only alphabets allowed of 2 Length)", "PRICE INCREASE APPROVER")

    Do Until (isString(Ret) And Len(Ret) = 2)
        Ret = InputBox("Please Enter Initials - (Only alphabets allowed of 2 Length)", "PRICE INCREASE APPROVER")
    Loop

    c.Value = UCase(Ret)
    '
    '~~> Rest of the code
    '
End Sub

Function isString(s As Variant) As Boolean
    Dim i As Long

    isString = True

    For i = 1 To Len(s)
        Select Case Asc(Mid(s, i, 1))
        Case 65 To 90, 97 To 122
        Case Else
            isString = False
            Exit Function
        End Select
    Next i
End Function

EDIT

I see one flaw in your approach. What if the user wants to cancel and exit? You might want to consider this code?

Sub Sample()
    Dim c As Range
    Dim Ret

    Set c = Sheets("CompilePriceAdjustments").Range("E2")

    Ret = InputBox("Please Enter Initials-(Only alphabets allowed of 2 Length)", _
          "PRICE INCREASE APPROVER")

    '~~> Added  Or Ret = "" so that user can cancel the inputbox if required
    Do Until (isString(Ret) And Len(Ret) = 2) Or Ret = ""
        Ret = InputBox("Please Enter Initials-(Only alphabets allowed of 2 Length)", _
        "PRICE INCREASE APPROVER")
    Loop

    '~~> This is required so that user can press cancel and exit
    If Ret = "" Then Exit Sub

    c.Value = UCase(Ret)
    '
    '~~> Rest of the code
    '
End Sub

Function isString(s As Variant) As Boolean
    Dim i As Long

    isString = True

    For i = 1 To Len(s)
        Select Case Asc(Mid(s, i, 1))
        Case 65 To 90, 97 To 122
        Case Else
            isString = False
            Exit Function
        End Select
    Next i
End Function

Answer 2

Consider:

Sub dural()
    Dim c As Range, init As String
    Set c = Sheets("CompilePriceAdjustments").Range("E2")
    init = ""
    While Len(init) <> 2
        init = Application.InputBox(Prompt:="Enter two initials", Type:=2)
    Wend
    MsgBox "Thanks"
    c.Value = init
End Sub