I am not an expert in Excel or the Automation of Excel but, why are you applying a formula to an object like a TextFrame? I would think a cell would be best suited for a formula.
When I look at your approach to writing code, I notice that you are not trying to instantiate new objects. This is going to limit your ability to rely on intellisense and debug. Adding lines of code will help to isolate issues when trying to incorporate new methods and adjusting properties.
I was able to get the following code to work and add text to a TextFrame but do not see the advantage of this.
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Code:
Dim xlApp As Excel.Application
Dim myBook As Workbook
Dim mySheet As Worksheet
Dim myShape As Shape
Set xlApp = CreateObject("Excel.Application")
xlApp.Visible = False
Set myBook = xlApp.Workbooks.Open("C:\Test\" & "TestAutomation.xlsx")
Set mySheet = myBook.Sheets.Item(1) 'Grab the first worksheet
Set myShape = mySheet.Shapes.AddTextbox(msoTextOrientationHorizontal, 171, 135, 72, 72)
myShape.TextFrame.Characters.Text = "'Auto Submit Calculations'!B3"
myBook.Save
myBook.Close
xlApp.Quit
Set myShape = Nothing
Set myBook = Nothing
Set mySheet = Nothing
Set myChart = Nothing
Set xlApp = Nothing