My Chipwhisperer times out when I try to capture data

Check that you did all the steps:

import chipwhisperer as cw
scope = cw.scope()
scope.default_setup()
target = cw.target(scope, cw.targets.SimpleSerial2)
target.baud = 9600
scope.arm()
target.ser.write(b"coucou\n")
ret = scope.capture()
trace = scope.get_last_trace()

Also check if your target actually answers your message with an oscilloscope or logic analyser.

The logic analyser cannot be open on my Linux

Install the package sigrok-firmware-fx2lafw

My target never answers my messages

Don't forget to append a '\n' after your password attempt.