If you want to unlock Sebulba or Roger, you’ll need to complete the Sacred Textbooks side mission first. You can start it in Theed, on Naboo. You can get it from the Aspiring Scholar you meet in the Scholar’s Residence. He will ask you to locate three Sacred Textbooks.
Sacred Textbooks Locations in LEGO Star Wars: The Skywalker Saga
The first Sacred Textbook is located in the same building you need to open in the Locked Out side mission. The password to that house can be obtained by interacting with the terminal in Corellian Cantina, found in Coruscant’s Uscru District. After completing the Locked Out mission, you can return to the building to grab the Sacred Text.
Coruscant and Ahch-To
The other two texts are located on different planets, namely Ahch-To and Coruscant. On Ahcht-To, you’ll need to look inside a small cave in the southern portion of the map.
The last texts is inside the Jedi Temple Coruscant. You can access the temple by catching a taxi in the Federal District. In the temple, you’ll need to look in the library. You need to talk with one of the NPCs found in this location first, though.
After getting all of the texts, all you need to do is return to the scholar on Theed to complete this mission.