If you embed the Livestream player on your website and notice that you are unable to switch it to full screen on a desktop, it is likely due to your iFrame embed being inside of another iFrame on your website. Edit your website to ensure the Livestream iFrame and any other iFrames or wrappers are separate from each other.
Unfortunately, if you are embedding your event on a website developed on Wix, there is no workaround for this issue due to HTML constraints.