Switch on off using triac BTA24, When off keep flashing LED lamp
Your Triac has excessive leakage and the LED AC_DC SMPS charges up and then discharges and repeats like a unijunction relaxation oscillator.

Either reduce the leakage or reduce the load impedance to the LED with a 4W AC bulb (Xmas or chandelier light) or put an X-rated plastic film cap 0.022uF to 0.047uF in parallel with the LED or triac load output to shunt some leakage which may not be enough.