The best way to start the license server is to launch the License Administrator and select File>License Tools Services.
Press the Start button for sesinetd.

Note that there was a bug in earlier versions of Houdini where the start did not work properly. Ensure you are using a build of Houdini newer than 20.5.527.

You can also use the Terminal to start the license server:

To start the license server daemon (sesinetd) launch the Terminal from Utilities and type:

sudo launchctl start -F com.sidefx.sesinetd

You will be prompted for a password. This is your password for the machine.
Note that you won't receive any feedback about your password until you hit the Enter key.

To verify if the license server (sesinetd) is now in the running state type:

sudo launchctl list com.sidefx.sesinetd

If there is a process ID (PID) listed in the output from the command then the license server daemon (sesinetd) is now running and you can try launching Houdini. The process ID (PID) is a unique number on your system and not the number listed in the example. Each time you start sesinetd, it will be given a different process ID (PID).

The output of the list command above should look like the following:

sudo launchctl list com.sidefx.sesinetd
Password:
{
    "LimitLoadToSessionType" = "System";
    "Label" = "com.sidefx.sesinetd";
    "OnDemand" = true;
    "LastExitStatus" = 0;
    "PID" = 17455;
    "Program" = "/Library/Preferences/sesi/sesinetd";
    "ProgramArguments" = (
        "/Library/Preferences/sesi/sesinetd";
        "-D";
        "-l";
        "/Library/Logs/com.sidefx.com/sesinetd.log";
        "-V";
        "2";
    );
};