How do Barcelona tickets work/where to buy?

I'm trying to buy tickets for the game on January 21st and I cannot figure out what's going on. The game isn't listed on the ticket section of the FC Barcelona website. I do see tickets on stubhub but I don't know how to tell which side is the Barcelona side and I worry about the tickets being legit when the FC site isn't selling them. I went to one game in the past and the FC site didn't sell tickets until last minute so I thought this was the case. To test that theory I was following what was going to happen for the game on the January 8th but they still didn't post tickets on the FC Barcelona website so I don't feel like waiting will result in getting tickets. Does anyone know what's going on or how this works?