If you are on AML-S805X-AC, you need to use i2c-gpio as pin 3 and 5 are not hardware I2C per the schematic: GPIO Pin Header Maps and Wiring Tool for Libre Computer Boards
If you are on AML-S805X-AC, you need to use i2c-gpio as pin 3 and 5 are not hardware I2C per the schematic: GPIO Pin Header Maps and Wiring Tool for Libre Computer Boards