Follow the brake rod onder the mowe to the brake on th transmission.
1) make sure it is not caught on the frame anywhere
2) if the brake lever is locked on then undo the nuts either side of the caliper on the transmission.
DO NOT TOUCH THE MIDDLE NUT WHICH THE LEVER PIVOTS ON.
That is the adjustment
Pull the entire brake caliper off the transmission.
Place a catch box under the brake because there may be two shims which are loose & will fall out
Pull the disc off, note there is a pad & shim behind it as well.
Clean everything and in particular the two pins that the brake lever push against to make the brake work