A Unity ID allows you to buy and/or subscribe to Unity products and services, shop in the Asset Store and participate in the Unity community.
Separate names with a comma.
A simple restart of Unity seemed to fix it for now for me.
Any more info on this? I got this on 2019.2.18f1 today with Timeline 1.1.0. First time trying out timeline so a little at a loss at where to go...