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.
This worked for me.
UPDATE: Changing my username to something else worked. I can now access and use Unity Play normally. That seems to be the band-aid fix to this...
Mission: Explore Unity Tutorial: Publish your project Unity Learn won't accept the link to my published WebGL project. I had to publish it...
I'm currently experiencing the same issue without ever having changed my username.