The most frequent reason for this is the Agent being behind a proxy server, or a firewall blocking outgoing requests from Agent to server.
Make sure to configure your firewall and/or proxy server as described in 0patch User Manual (https://0patch.com/user_manual.htm) in section "Network Connectivity".
With agent version 18.104.22.16850, there's another possible reason for seeing this error: we changed the scheduling logic of automatic syncing, which turned out to have an undesired side effect. Namely, when 0patch Service starts (upon starting up the computer, or waking it from sleep/hibernation), it immediately tries to sync but networking may not be completely set up by then, which leads to the above message. Now, previously the agent would have re-tried within a minute (which would remove the message if online) but now it waits for 60 minutes, and during this time you see the message and the yellow 0patch icon. While this bug merely delays your agent's first successful sync after startup for 60 minutes, it gives an impression of something being terribly wrong so we're fixing this in the next agent version.