I get similar results when trying to add Draft to the end faces too. Instead of using Draft you can crate a Sketch of a triangle on a side face, then Pocket "Through All". If you make the triangle legs the same length as the side of the tube steel and the Pocket fails, try extending the triangle legs outside the solid (it's become habit for me):
Word size of OS: 32-bit
Word size of FreeCAD: 32-bit
Version: 0.16.6707 (Git)
Build type: None
Branch: releases/FreeCAD-0-16
Hash: 5465bc47c95db45e0be85dc0e2872419efadce0f
Python version: 2.7.12
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 6.8.0.oce-0.17
OS: Ubuntu 16.04.2 LTS