Procfile is
web: npm start
Error log shows
app[web.1]: bash: npm: command not found
Package.json is being parsed fine, install works fine. What's the next step for diagnosing?
It's best to use node path/to/your/server.js rather than npm start as a passthrough. Node is guaranteed to be executable in the $PATH.
Try copy/pasting the script used with npm start into your Profile, and see if it works!