"Sagres" is located between Seville and Lisbon, so it's easy to access for players from Spain or Portugal.
However, players from France, the Netherlands, England or Venice first need to obtain the Port Permits for the sea area where Sagres is located to enter the town of Sagres. To do so, follow the Tutorial and visit different facilities within the city you first started in the game, until your "Experienced Sailor" starts talking about the "Rumored Training School." When the "Experienced Sailor" finishes talking about it, you will obtain the necessary Port Permits.
Also, you can take the "Liner" for Sagres by speaking to the Port Official in any Cities to reach Sagres easily.