recommend using an automated install script instead of manually working through the install steps yourself
(not that there’s anything wrong with the instructions, but there are so many places where things can go wrong, it can be difficult for folks here to help troubleshoot)
here are a few options if you’re willing to restart from scratch …
RoPieeeXL (https://ropieee.org/xl/) - best if you want an appliance-like experience for headless Plexamp in combination with other streaming protocols (Airplay, Spotify Connect, etc.) … this is an OS install image and all user configuration is done thru a web interface
if you’re brave, i put together a simple install script and some setup instructions (install and set up Plexamp headless on Raspberry Pi · GitHub) - the script is very basic, but even if you use one of the other install options, you might want to refer to some of the app configuration notes here