Well I’m not going to travel to Cuba for the sake of an internet discussion

You don't need to, it's very easy to find documentation on the subject for free online if you are just willing to actually learn about it.

But it is true that political opposition is forbidden and the only allowed party is the communist, right?

No it's not, not only are other parties and independent candidates allowed, but the communist party isn't allowed to present their candidates to the elections because candidates to the elections are not nominated by their party but by the peoples of their community, the communist party do end up having their members picked for the elections because the party is overwhelmingly popular but there is nothing to guaranty that any of them will get in.