When your Solr seems to be started properly (a process is running with the correct data directory) but never responds properly and replies (via the API or web interface) with...
404 "Not Found"

Rails Long Term Support

Rails LTS provides security patches for old versions of Ruby on Rails (2.3, 3.2, 4.2 and 5.2)

• Prevents you from data breaches and liability risks
• Upgrade at your own pace
• Works with modern Rubies

Read more Show archive.org snapshot

... check if Solr's log directory is actually writable for the user running it.