How to Unlock the Multiplayer
You must have defeated Administrator to go to chapter 2 of the story. (to get to chapter 2 it’s about +13-15 hours of gameplay)
-As soon as you are in chapter 2 before going on the first mission (Remember to go to Kirito’s tent and customize your character at the desk)
-In chapter 2 of the story, you will be asked to investigate a strange creature that appeared in an area. Go check and find out things ???, then go check in the cave at ??? and after that 3000 dialogues will start with a cutscene at the end with Alice, Eugeo, and Kirito. From there you unlocked the Multiplayer.
-Happy game in co-op.