If you open up the assignable unit within Lectora (in the left hand pane) you should be able to enforce pre-requisites (A "Pre-requisites" tab will appear when you have more than one assignable unit in a title).
If you are having trouble finding the assignable unit (AU), double click the Title properties and make sure that the title is scorm compliant (this option is found under the "Content" tab in the "Title Properties" dialogue box).
The navigation between the AUs is handled by the LMS.
Let me know if this helps.
~Mike
