Yes, as the course creator you can decide whether a sequence of lectures is optional or mandatory.
By default the sequence is optional and the learner is free to attend the different lectures of a course in any order.
Please, find the needed course and click Manage Course. Then click on Steps Configurator.
You can change the order of your course's steps with a simple drag & drop. Moreover, Schoox allows you to decide whether the sequence of the steps will be mandatory or not.