Gas pressure requires molecules to be in contact with each other, bouncing off each other, causing millions of collisions per second, etc… If you release gas into the vacuum of space, the first molecule that pops out will shoot off into the distance at a constant speed, so will the one behind that, never catching up with the first one. The third, fourth, etc… all fly off into the distance trying to fill the vacuum by finding their empty corner. So no matter how much gas you produce none of it will ever change the pressure under a space ship. None of it if will ever push a spaceship. To push a spaceship there must be some locally high pressure under it, which is impossible since the pressure in space is 0 everywhere.
I think you are still struggling to understand what happens in a rocket motor's combustion chamber, even though this thread is full of posts which have explained what happens with total clarity. Are you imagining a little "Pffffft...." noise as fuel is sucked out helplessly into the vacuum of space? Because that is not what happens.
When a rocket motor is operating, insane quantities of fuel and oxidiser are pumped into the engine's combustion chamber and ignited, creating an apocalyptic, roaring, barely controlled explosion of boom.
There is so much boom that even if the engine bell is venting to a vacuum, there is still more than enough boom to continuously fill the combustion chamber. In the case of the Saturn V's first stage engines, there was enough boom to create an operating pressure in the combustion chamber of 1,015 psi. A reminder of what this looks and sounds like might help:
" class="bbc_link" target="_blank" rel="noopener noreferrer">Note at around 2.44 plenty of "molecules in contact with each other".
Now, I know the first stage didn't operate in space - but thirty seconds with Wikipedia will tell you that, say, the J-2 engines in a Saturn V's seond and third stages were equally happy making boom in space.