The effects of this error message is that:

1. VAC is not enabled
2. the game server will not be listed in the server browser, which is a significant handicap to public servers.

If you just set up your server: don't panic! It's normal to see this at first, while your server connects soon after.

You can check for connectivity by searching the console for occurrences of VAC secure mode is activated. Afterwards, upon restarting the server you should see the following:

Connection to Steam servers successful.
   VAC secure mode is activated.

If after a few hours your server does not connect to Steam, here's a few things to check:

Stop the server, open the file [SrcDS Root]/config/config.vdf (create this file/folder if it does not exist), and write the following data to the file. Upon restart, the server should immediately connect.

"InstallConfigStore"
{
	"Software"
	{
		"Valve"
		{
			"Steam"
			{
				"CurrentCellID"		"81"
				"PingTimeForCurrentCellID"		"0"
				"CM"		"162.254.192.87:27022;162.254.192.101:27019;162.254.192.109:27017;162.254.192.71:27020;162.254.192.71:27027;162.254.192.101:27020;162.254.192.71:27025;162.254.192.101:27017;162.254.192.100:27020;162.254.192.87:27024;162.254.192.71:27021;162.254.192.71:27023;162.254.192.87:27021;162.254.192.71:27017;162.254.192.71:27018;162.254.192.87:27026;162.254.192.87:27023;162.254.192.71:27026;162.254.193.74:27023;162.254.192.100:27018;162.254.193.74:27026;162.254.193.102:27019;162.254.193.74:27019;162.254.193.74:27022;162.254.193.102:27020;162.254.193.74:27017;162.254.193.74:27018;162.254.192.108:27017;162.254.192.108:27019;162.254.192.109:27021;162.254.192.109:27020;162.254.192.101:27021;162.254.192.87:27017;162.254.192.109:27019;162.254.192.100:27021;162.254.192.71:27019;162.254.192.101:27018;162.254.193.102:27021;162.254.193.102:27017;162.254.193.102:27025;162.254.193.102:27026;162.254.193.74:27021;162.254.193.74:27020;162.254.193.74:27025;162.254.193.102:27018;162.254.193.74:27027;162.254.193.102:27022;162.254.192.87:27018;162.254.192.87:27025;162.254.192.109:27018;162.254.192.87:27027;162.254.192.108:27020;162.254.192.100:27017;162.254.192.108:27018;162.254.192.100:27019;162.254.192.87:27019;162.254.192.108:27021;162.254.192.87:27020;162.254.192.71:27022;162.254.193.102:27027;162.254.193.102:27024;162.254.193.102:27023;162.254.193.74:27024;162.254.192.71:27024;162.254.195.87:27017;162.254.195.82:27017;162.254.195.87:27024;162.254.195.87:27022;162.254.195.67:27017;162.254.195.66:27019;162.254.195.87:27027;162.254.195.71:27025;162.254.195.71:27023;162.254.195.71:27024;162.254.195.87:27020;162.254.195.71:27022;162.254.195.82:27018;162.254.195.71:27017;162.254.195.71:27020;162.254.195.67:27018"
				"CMVer"		"-1299633720"
				"NCTF"		"0"
			}
		}
	}
}

• [SrcDS Root] refers to the install directory of Source 2007 Dedicated Server; it does not refer to the gesource or steamcmd folders.
• It is important to quit the server before modifying the file, because when quitting, the server will write out an empty/invalid config.vdf.

While there is not a complete and reliable list of Steam endpoints, it is known that you will need to allow the following ports for outbound connections:

TCP ports 80 and 443

TCP ports 27015-27030

UDP ports 27015-27030

It has anecdotally not been very helpful, but a last-resort option is to check the connection log for your server, which is located at [SrcDS Root]/logs/connection_log_[port].txt