I'm not educated on the subject. Why can't Cuba import the fertilizer and build the machines? There are open source blueprints, like the Global Village Construction Set.
Cuba is an island with very little natural resources. Therefore, to build almost anything, it needs to buy it from other countries. The sanctions makes everything coming from outside much more expensive. And to buy things, they need money (dollar, or some other international currency). They can get it only in the few areas where they can be internationally competitive (as any imported product is much more expensive to them, they will not be competitive in most industries): tourism, offering medical services and selling cigars.
As they have fewer baskets to place their eggs, they are much more succetible to crisis: a blow in one economical activity generates crisis that affects a lot all other areas. For example, one of the causes for the current crisis is the pandemics that dropped their tourism to zero for 2 years. Now they have much less to invest, affecting all economical activities.
Foreign currency the US is Cuba's most logical export market. So, with a shortfall of dollars/etc, importing things is problematic. So Cuba's reliance on importing food - especially from the US - looks like a spiral: eating today taking precedence over planting etc.
I don't know as much about this next part, but seeds are also harder to import. I'd hazard a guess at there not being much capacity in that market unless you're buying from Monsanto or the like, which seems not to be an option in Cuba as those (American) companies are not exempt from the embargo, unlike exporters of food and medical supplies.
The answer here is "monsanto is a de facto monopoly" - they're essentially the only company on earth that this kind of business can be done with on any kind of scale.
As they have fewer baskets to place their eggs, they are much more succetible to crisis: a blow in one economical activity generates crisis that affects a lot all other areas. For example, one of the causes for the current crisis is the pandemics that dropped their tourism to zero for 2 years. Now they have much less to invest, affecting all economical activities.