Nothing's wrong — this is normal.
The MQ-135 gas sensor (an add-on module) needs a short warm-up after long storage. The first time you use it, power on your Butter Bot with the module plugged in and let it warm up for 5–10 minutes before relying on readings. During that time it won't react to CO2 or other gases.
You only need to do this once — after that it's ready instantly, even if you use it months apart.
And you don't have to remember it either: Butter Bot will tell you when it's still warming up — for example, "Gas sensor currently calibrating, but the result is XX."