In your specific instance you use only a single index for a two-dimensional variable matrix, causing the KeyError.
You should loop over the index sets from your model to produce the results, so for example:
outputVariables_list = [model.variable_heatGenerationCoefficient_SpaceHeating[building, time_slot], model.variable_temperatureBufferStorage[building, ...
I don't know your entire code or the errors you get, but
the general way is
optimal_values = [value(model.x[key]) for key in model.x]
df = pd.DataFrame(optimal_values)
is your target variable that is optimized.