Sub t()
''' Retain every first character of each cell in Table 1


Dim oRange As Range

With Selection
    Set oRange = .Range
    With oRange
        .Start = .Start - 1
        .Delete ''' Run-time error 5904: Cannot edit Range
    End With
End With

End Sub

Dim str As String
Dim oRange As Range

With ActiveDocument
    With .Tables(1)
        For Each Cell In .Range.Cells
            With Selection
                Set oRange = Cell.Range
                With oRange

''' It seems that for table cells .Start cannot function
''' First we should .End = .End - 1 first

                    .End = .End - 1
                    .Start = .Start + 1
                    .Delete
                End With
            End With
            
        Next
    End With
End With

Sub Demo()
Dim oCel As Cell
With Selection
  If .Information(wdWithInTable) = False Then Exit Sub
  For Each oCel In .Tables(1).Range.Cells
    oCel.Range.Text = oCel.Range.Characters.First
  Next
End With
End Sub