"File In Use" Message - how can this be disabled?

Hi everyone,
Thank you for your tips. I have tried them all but I'm still getting the "File In Use" notification.
However, I have found the cause of the problem - but I'm still after any advice if you have some?

In my excel workbook I have got an ActiveX Control Button ("Forms.CommandButton.1") which calls my vba code through the below:

Private Sub SubmitAssessment_Click()
'Application.Run "SubmitAssessment.Submit_Assessment"
Application.Run "SubmitAssessment.tst"
End Sub

This is causing excel to display the "File In Use" notification.
If I add in a Form Control Button instead, this is assigned to the Macro directly "Submit_Assessment" and it doesn't display the "File In Use" notification.

My problem now is how do I assign a private sub macro to a Form Control Button?