The chances are if the atmosphere is only CO₂, then there are no photosynthetic life forms near or on the surface. But there could be anaerobic bacteria life forms that do not need oxygen and can get on well. The problem with CO₂ is that it is a good greenhouse gas. This means that an atmosphere made up of a lot of CO₂ will trap most of the solar radiation and make the planet's atmosphere heat up.

There are no planets made of carbon dioxide. The planets that have significant amounts of carbon dioxide in their atmospheres are Venus and Mars.

Life as we know it cannot exist on Venus. It could exist on Mars; protected below the surface where water can still be liquid (it's a pressure-thing). Just because it could be there does not mean that it is, however, and thus far the evidence of Martian life remains ambiguous.