You are passing the whole WorkSheet object
, instead of just its name. Try this:
Dim LastEntryCol As String
Dim ShtName As String
ShtName = ThisWorkbook.Sheets(2).Name '****The number 2 will need to be changed to an index. For now it's hard-entered.
LastEntryCol = Application.InputBox("What is the letter of the last column containing entry labels?", ShtName)