Hello and welcome.
The service brakes system warning can come on for a few different reasons.
A partially depressed parking brake.
A low fluid level in the master cylinder, or the level sensor itself.
A problem in the ABS system.
The service brake system warning has nothing to do with worn pads / replacing pads. It is not an indicator of brake pads needing replacement.
It is possible that the park brake is on, or the fluid level got low because of the worn pads, and now the level sensor is stuck.
Check the park brake, and tap on the master cylinder with the key off. Then turn the key on and see if light goes out.