Do you need an adapter to put freon in a 1999 Chevy lumina?

Yes, you almost certainly need an adapter to recharge the AC system in a 1999 Chevy Lumina. The low-side service port on vehicles of that era typically uses a R-134a fitting that's different from the standard can tapers found on most recharge cans. You'll need an adapter to connect the can to the vehicle's fitting. These adapters are readily available at most auto parts stores.