Refuelling/tire change seemingly not possible during races with weather changes add-on
In this case the speed of your car is probably reduced to the minimum by the weather add-on. If you now enter the pits and cross the pit lane sensor, the control unit receives the signal that you’re now allowed to refuel. But if you now press your controller more than 50%, the control unit will “think” you’re back on track and will revoke the permission to refuel. This is why the “refuelling allowed” is immediately revoked after you crossed the sensor.
To avoid that, you whould either use a higher minimum speed in the weather settings, or make sure to be careful when entering the pits to not press your controller with more than 50%.