This question already has an answer here:
- Intercity transportation in Cuba 3 answers
- Should I book Viazul bus tickets on Viazul.com in advance?
- There are not so many departure times - can tickets be sold out if I will come to a bus station and will try to buy a ticket few hours before departure?
- Is it possible to reserve a ticket in a station for a route which is between different cities than station?
There is similar question about Intercity transportation in Cuba but it is more about specific routes and Viazul alternatives - taxi colectivos.