No, radon is more an issue in a basement with no air flow, where the gas can accumulate. A greenhouse will have plenty of air flow so radon would be no more an issue than if you were standing outside.
The holy trinity of wholesomeness: Fred Rogers - be kind to others; Steve Irwin - be kind to animals; Bob Ross - be kind to yourself
Look! I laid an egg! Why does it smell like that? Tiny ad, does this smell weird to you?
Contemplate spending the next 20 years puttering in permaculture projects