sudo systemctl restart wg-quick@wg0 exited with error code

When
sudo systemctl restart wg-quick@wg0
fails with an error code, it indicates that the WireGuard service encountered an issue during startup or configuration application. 

Immediate Diagnostic Steps

To find the specific cause, run these commands:

Detailed Status:
sudo systemctl status wg-quick@wg0.service
System Logs:
journalctl -eu wg-quick@wg0.service
to see the exact line causing the failure.Manual Test: sudo wg-quick up wg0 often provides clearer error messages than systemctl. 

Common Causes and Fixes

Error Message / Symptom Likely Cause & Solutionwg0 already existsThe interface was not shut down properly. Run sudo wg-quick down wg0 before restarting.RTNETLINK answers: Permission denied / Operation not supportedMissing kernel headers or TUN/TAP device. Run sudo apt install linux-headers-$(uname -r) or enable TUN in your VPS panel.Line unrecognized / Configuration parsing errorSyntax error in /etc/wireguard/wg0.conf. Check for stray characters, missing headers like [Interface], or incorrect comments (use #).resolvconf -d / Unknown interface 'tun'Conflict with DNS management. Install openresolv or resolvconf if your config uses DNS = ....Status 127Missing wireguard-tools or wg-quick binary. Reinstall with sudo apt install wireguard.