It sounds like the hammer strut isn't lined up with the mainspring cap. This is the typical problem most people have the first few times so don't worry about it. You need to hold the barrel up when you close the mainspring housing.
Release the lever and let the mainspring housing swing out. Hold the barrel straight up and see if you can see the hammer strut hanging down. You close the housing, still holding the barrel up. That little strut then lines up with the top of the mainspring housing. You should feel a little tension on the housing when you've almost got it closed. Then you can snap the lever shut.