How to draw rectangle with Shape Digitizing toolbar in QGIS 2.10.1?
This feature was introduced with QGIS 3. If there is no particular reason why you use this old version of QGIS I'd like to, at first, suggest to update to QGIS 3.4 or at least to QGIS 2.18.
However, in QGIS 2 you can use a plugin called
Rectangles Ovals Digitizing: https://github.com/rdbath/RectOvalDigitPlugin
On the plugin site it's stated that now
CADDigitize plugin should be your choice, however, I never tested this one: https://gitlab.com/lbartoletti/CADDigitize/