VAC stands for Valve Anti-Cheat and is made so that your server can stay secure against hackers and cheaters. However in some cases on your private server you may need to disable it for mods or playing around with cheats. You can use these steps to disable VAC on your server.
1. Open the File Manager
2. Right click start_server.sh
and click Edit
3. After usercon, add the -insecure
parameter.
4. Save the file and Restart the server