That should be it. I would try turning off the install from other sources, reboot, turn on other sources, and try develop mode again. At the very worst you can reflash.

Also be aware of some kinda obvious stuff you probably already thought about but just in case:
You have to be connected to the internet
It might be switching off while it installs

I'd do the above reboot, make sure you're on wifi, and trying again. But only try it once and leave it alone for awhile.