In planned economy, some burreaucrat decides it will be created 10 million tons of potatoes in the next 5 years. Not less can be made (or you get in prison) nor more can be made (prison again).

Now what if the demand rise to 15 million? Sorry, we have a plan we must only grow 10 million. You may think it's technically easy to just sow more fields next year, but we already have plan for fertilizers, tractors etc. for only 10 millions. You can't grow more even if you want.

And we are talking about growing food, which is extremely volatile and dependent on weather. One bad harvest and millions will starve.

In my country during communism, there was a shortage of ketchup. There was a lot of tomatoes, actuall, they had to waste the harvest. Becuase there was a shortsge of plastic bottle caps for ketchup.

See? In market economy, someone would jump on the train immediately and make the caps in a few weeks. In planned economy, you couldn't make more and the whole harvest was wasted.