Inside your Epson L121, there are physical sponges called waste ink pads. These catch excess ink during head cleaning and printing. The printer has a built-in counter that estimates when these pads are full. Once that counter reaches 100%, the printer locks itself to prevent ink from overflowing and damaging the internal hardware.
If you are looking for "deep" cleaning or quality adjustments rather than a counter reset, use the built-in Epson utilities: adjustment program for epson l121