In a free app, you can view the Live Logs while the app is starting up. On the Environments tab, click View Live Log. Be sure to open Live Logs before trying to deploy. I think the live log will show you some information that points you to the issue.
Hope that helps.
Note that with Live Logs, you will not get log files. Log files are not available in the Free mode. Instead, you will get a separate browser tab that looks will display log information. It will look something like this:
I have tried to view live logs while publishing but nothing is generated in the logs section. Please see below screenshot