At this level (alpha build 1) I can't imagine not being able to colonize a planet as long as it is listed as habitable. I have had the same problem many times, sometimes I am able to colonize it next turn without completing any research, sometimes it takes a few turns, and now I have one I can't seem to colonize after several turns after trying it with several ships.
I checked with the devs and they see this as a bug. Do you wish to report it to support, or should I?