A player runs its own network checks and reports which one failed. Each error points at a different layer, so the message narrows the search considerably.
Wired connection
The player cannot establish a link or get an IP address over ethernet.
Check the cable at both ends, including whether it is fully seated rather than partly inserted. Then check the switch port: the player may need allowlisting on your network, or the port may be on the wrong VLAN.
Wireless connection
The player cannot connect over Wi-Fi.
The usual causes are a wrong password, a wrong or unfindable SSID, a band the board does not support, an access point that is out of reach, or a player that needs allowlisting.
Most of these come from typing network details by hand into a configuration file. Use a USB keyboard instead where you can, because it lets you pick the network from a list rather than spelling out the SSID.
Time server (NTP) connection
The player cannot reach a time server, so it cannot set its clock.
Check that port 123 is open and that ntp.ubuntu.com is allowed. Then check for an error in a manual NTP configuration, or a routing problem reaching the server.
Accurate time is not cosmetic here: security and scheduled playback both depend on it. See Configure time servers.
Screenly backend connection
The player cannot reach api.screenlyapp.com, mtls.screenlyapp.com, or ws.screenlyapp.com.
Check whether those hostnames are blocked by your firewall, whether there is a DNS mistake in a manual network configuration, or whether the player is on the wrong VLAN.
Operating system connection
The player cannot reach the resources Canonical provides for Ubuntu Core.
Check the hostnames and ports are allowlisted in full, including the Ubuntu entries, and that the player is on the right VLAN.
If an error persists
Once you have confirmed the requirements are in place, power cycle the player: disconnect it for 20 to 30 seconds and reconnect. If it comes back with the same error, contact support.