Why are Ainz Ooal Gown troops attacking lizardmen in Overlord II (currently ongoing)? Please explain.
They want to subjugate them
Ainz Ooal Gown is trying to expand their power, so gaining the extra land and bringing the lizard-men under control (or annihilating them) beings a small degree of extra security and power to Ainz Ooal Gown. Admittedly it is a huge amount of effort for essentially no reward (the lizard-men are very noticeably inferior). But it is also being used as an experiment/training exercise for Cocytus.
I want to expand on Jesse's answer.
Indeed there was reason to attack them in regards to land, power, and fame among other things that taking over or destroying the Lizardman would bring, but there was one other pretty big thing Ainz wanted to achieve, Though it has not really been shown in the Anime just yet (The major clues should show up in the next few episodes), it was months ago in the manga and years ago in the light/web novels.
All of the followers in Nazarick follow Ainz's orders almost to the letter. Outside of ones like Demiurge and Albedo who are also quite intelligent and in Albedos case, somewhat rebellious due to her love of him, they take his words as absolute truth, and orders are to be absolutely followed. Their loyalty to him is too perfect, so its restricting and requires a lot of orders. The only time they would go against his words is if it was to make him seem more like the awesome and unbeatable leader they view him as.
Cocytus was given orders to subjucate the Lizardmen, while at the same time given numerous restrictions that made it extremely difficult for him to succeed. Cocytus acknowledged that he was unlikely to win, but went ahead anyway and tried before failing. The goal was to help Cocytus realize his shortcomings with completely following Ainz orders every single time, and to help him grow as a commander by realizing why this situation was so difficult and how he could have solved it.
So what his true goal was, was to both help Cocytus learn how to win a difficult battle, and secondly to help the guardians to better interpret and question his orders so that they would follow the intentions rather than the words themselves. Cocytus somewhat learns this by his suggestion to (not yet seen in the anime, but found in manga chapter 22/23)
To not exterminate the Lizardmen, but instead to subjugate them.
Ainz knows he is not perfect, but the guardians do think he is, to a dangerous level. Though it may be a ploy, Demiurge constantly suggest good ideas that Ainz doesn't seem to think of, and then says that Ainz was implying it or waiting for someone else to come to that conclusion as a test. Regardless, everyone else seems to agree that Ainz thought of these ideas first, not Demiurge due to his praise. He wants to lessen that to some degree so that the Guardians stop saying his decisions are all that matters, and so that they contribute their own thoughts to discussions.
As a shining example, in chapter 27, Ainz has 2 options on how to deal with a specific issue with the Lizardman (see the next spoiler), and with only 2 options, he asks all of the guardians present what to do, and everyone says that they will agree with whatever Ainz decides to do.
The situation being on weather to test the Resurrection wand on any of the dead leaders of the Lizardman tribe, since Cocytus killed all of them except the White female Cruche in a duel to decide if they would be under Ainz's rule or not.
Just adding to the excellant answer given by @Jesse since I don't have the reputation here yet to comment.
Ainz said that his greatest power is his ability to control the dead, I think that was in episode 1 of this season. So, by annihilating the lizardmen he will be able to raise them from the dead to add to his troops, i.e. total subjugation.
This is all part of the plan to increase forces to deal with the pending conflict with the slain Theocracy, this was alluded to in episode one as well. This is ultimately why he is going after the lizardmen. There are other factors like spreading the name and reputation of Ainz Ooal Gown and the lizardmen villages are relatively close and their numbers and strength expected to be low enough to easily overwhelm. It didn't quite go as Lord Ainz had planned he said in episode three.