Precursor Quarantine

You need Atmospheric Cleansing to colonize that planet, which is normally researched in the Age of Ascension under the Planetology line of techs. It should tell you in the tooltip when you hover your mouse over it.

A side note is that the different ages are reached by researching technologies, so you really want/need to do a lot of research to open the Age of Ascension locked techs. The tech tree shows a progress number (20/32) towards the next age.

As far as I can tell, what techs are researched doesn't change the progress towards the next age, just the total number.

It can be frustrating to have a colony ship waiting there and be unable to use it.

I have atmospheric cleansing...that is the weird thing. I ran down that tech tree line because there was initially a red notice on the planet indicating the need for it. But the quarantine thing is in orange and I can't seem to figure out why I can't land.

If you can mouse over the planet and upload a screenshot, that could help us determine if the issue is something simple or worth reporting to Stardock

Well, problem solved. For some reason, the colony ship, despite having the colony module, didn't get the Ability to Colonize planets. After taking off the module and then reinstalling it at the shipyard, the ability appeared on the ship and it worked fine.