I got this by installing oTree on Render:
You have oTree 6.0.0a9. The newest version is 5.10.3.

I just changed the requirements.txt to:
otree>=5.0.0a21,<5.11
This way it forced back the version 5.10.3. But this will prevent me to get newest version in the future.

Is this expected? Or am I doing something wrong with the installation? Maybe wrong python version?