for a planet not to have an element expecially one as common as carbon would be extremly rare, and you would think the sensors on the starship would detect if a planet did not have any carbon, and would quickly rule out that planets canidecy for colanization
as for hydrogen, well that is the most abundent element in the universe, for a planet not to have hydrogen is just plain ludacris
now some heavyer elements a planet may be dafissant in, but mabe later in the game you can have astroied mining or somthing to get those (such as iridum wich is very scarse on earth, but fairly abundent on astroids)
all that said, it all boggs down to on simple statement, where there is a will, there is a way
if the colonists need plastic t survive, they will find a way to make plastic
its funny, I thought of the same question when I was reading the noliva, but the fact that eden has made plants grow on New Terra convinced me that if the soil had suffecent carbon/nitrogen/water for plants (even ganeticly enhanced ones) than it should have more than enough to make plastic for a few buildings and vecs