I have a late 2014 mac mini. WiFi works great but ethernet does not work. Keep getting the message: "Ethernet has a self-assigned IP address and will not be able to connect to the internet."

That doesn't necessarily mean that your Ethernet hardware is not working, actually the opposite. What it means is that whatever you're using as a modem has been queried for a workable IP address and somehow hasn't provided one. Many times ISPs purposely configure their networks to operate that way to prevent Internet access theft.

I would suggest contacting tech support for your ISP, resetting your modem, follow any guidance offered by your ISP.

Please note: When you ask a question like this and provide no useful information, i.e., the name of your ISP, the make and model of your cable or DSL modem, you miss the opportunity to get specific solutions to your issue: Writing an effective Apple Support Commun… - Apple Community