Based upon a similar question on the Microsoft Answers web page, their Skype Community Moderator explained what needs to be done: On the other hand, if this is the Skype UWP app those options won't be available.

Preventing Skype for Windows 10 (Skype UWP) from starting Uncheck the option to Start Skype when I start Windows on the right.Highlight the General settings section on the upper-left.

With the classic Skype application window open, do the following: Preventing the classic Skype application from starting