Machine will not connect

You see

You select a port in Laser Control (or Connection tab of Device Settings), click Connect, and either:

The status stays Disconnected.
The status flashes Connecting / Validating then drops back.
An error pops up.

Beam Bench opened the serial port and either sent a reset and got no response, or could not open the port at all. Common causes: another program holds the port, the driver is wrong, the port is right but the machine is unpowered, baud rate is wrong.

Fix

If you picked the wrong port

There are sometimes multiple serial ports visible. Try each one. The right one is usually named for the chipset (usbserial-1410 on macOS, CH340 or CP210x on Windows Device Manager, /dev/ttyUSB0 on Linux).

If another program has the port

Close any other program that talks to your controller (Arduino IDE Serial Monitor, other GRBL senders, anything that opened the port). Only one program can have a serial port open at a time.

If the machine is unpowered or the cable is bad

Confirm power, swap the USB cable for a known-good one. Cheap USB cables sometimes do not carry data (charging-only).

If the baud rate is wrong

See Baud rate mismatch. Most GRBL controllers default to 115200.

If the driver is missing (Windows)

Open Device Manager. If you see "USB Serial Device" without a recognized driver, install CH340 or CP210x driver based on your controller. See Windows driver issues.

If permission denied (Linux)

Add yourself to the dialout group:

sudo usermod -aG dialout $USER

Log out and back in.

Still stuck?

Open the Connection Diagnostics panel and watch the events log during a connection attempt. The failure stage tells you what gave up.
Try connecting with a different tool (Arduino IDE Serial Monitor at the correct baud) to verify the cable / port / driver are good.
Post in the Facebook group with the diagnostics snapshot.
Send a connectivity feedback report from the Connection Diagnostics panel → Send to Beam Bench.