Why can’t I insert my water tank back into the unit?

You need to make sure the water tank is inserted in the unit to the right side of the machine. If you can't insert it , it may be that the pump system is in the way. In this instance, gently move right the spring lever and push upward on the pump system until the spring lever pops out to support the pump. After doing that you should now be able to insert the water tank.