Your exception tells you that another IWebElement is targetted by your search. "button" class is obviously too common in your code. Try to combine this class with another one around your target. It's seems that "clsaddbtn" can do the job but I'm sure you will need another one again because I see somes tabs in your browser that maybe hide the same classes.
You can look to PageObject model and use FindBySequence to reach your element.
And another point : Thread.Sleep is evil. If you must wait for your elements, you can use Selenium Wait, because if your server is busy, you will have free exception because your Thread.Sleep will not be enough, and maybe your Thread.Sleep is already too long and you are waiting for nothing.
[FindsBy(How = How.Id, Using = "elementId", Priority = 0)]
[FindsBy(How = How.Id, Using = "childElementId", Priority = 1)]
public IWebElement thisElement;