I am returning to WoW for dragonflight and I have a low level rogue. I tried to equip a sword in my main hand and a sword in my off hand and it’s not letting me do it. Rogues no longer able to dual wield?
Also what happened to the combat talent tree? They have added something called “outlaw” and it doesn’t look anything like combat.
dual wield is still there.
Outlaw is combat with a few more extra abilities and buttons.
Combat was replaced by outlaw because Blizzard thought that combat was thematically weak. So they went with a pirate style spec, in terms of playstyle it relies on random buffs from roll the bones, and build and spends combo points really fast which feeds into its passive ability to reduce the remaining cooldown on some of your abilities based on the amount of combo points you have spent. Its simple but very fast paced. You can still dual wield sword, but sin and sub both use daggers mostly, I think you can play sub with a sword in the offhand but sin has to use two daggers.
Ok thank you for the clarification.
Also, some of Outlaw’s abilities require a non-dagger in the main hand.
The only reason you wouldn’t be able to equip two 1h swords is probably because they are both main-hand required. Or the one you are trying to put in your off-hand is. I suppose the reverse could be the case, as well. You are trying to put an off-hand only sword in your main-hand. I am pretty sure those kind of swords are super rare these days. But, with Chromie time, who knows where your Rogue is. 
Many ask themselves the same question from the Legion expansion xD