This is due to older versions of Unity 2018.4 not supporting packages with optional dependencies on Timeline.
If you upgrade unity to the latest 2018.4 LTS version (currently 2018.4.15f1), this problem will go away.
For earlier versions of Unity, CM 2.2.9 must be used.
This is an older problem. But I made a video on this for myself, because I couldn’t find one. So sharing it with you guys. If it helps.
Youtube: https://www.youtube.com/watch?v=oQYvSXPzS94