Why is it so difficult or not possible at all to push down the lever of my Cremina?

Perhaps there is too much coffee powder in the portafilter. Check the amount of coffee in the portafilter. Check the grind coarseness as the coffee powder might be ground too finely. The ideal grind coarseness is attained if the extraction time for 0.85 to 1.01 oz. espresso is between 20 and 30 seconds.

Caution! Wait until the pressure is released -- the lever should be in the lower position -- before removing the portafilter.