You should refer to the name of the parent of the current form, that is:
DoCmd.Close acForm, Me.Parent.Name
The second argument of DoCmd.Close takes a string.
You can also loop through the forms collection.
FormsCount = Forms.Count - 1
For i = FormsCount To 0 Step -1
If Forms(i).Name <> Me.Name Then
DoCmd.Close acForm, Forms(i).Name
End If
Next