I am using TiledSharp for a quite long time now. This error usually appears if your created map does not contain such objectgroup.
Check your .tmx file if the objectgroup actually exist. it looks like this:
<objectgroup name="Obstacles">
<object id="1" x="35" y="39" width="24" height="20"/>
</objectgroup>
the code for this looks like that (in my case):
foreach (var o in curMap.ObjectGroups["Obstacles"].Objects)
collisionObjects.Add(new Rectangle((int)o.X, (int)o.Y, (int)o.Width, (int)o.Height));
Be aware that the name is case sensitive.
Objectgroups are usually at the end of the .tmx file