from the Bosch Service Manual:

DISHWASHER TEST PROGRAM ERROR CODES (on 2-digit digital displays):

0 – No faults

1 – Aqua Sensor (Sensotronic) fault

2 – Heating system fault (heater, Hi-Limit, flow switch, NTC, control module heater relay)

4 – Water filling fault

8 – NTC (temperature sensor) fault

16 – Water switch fault

Fault codes add up for multiple faults

(e.g. heating + water filling fault = 2 + 4 = 6)

I'm not a Bosch master, but it looks like 21 would be 16+4+1

Is there enough water in the Dishwasher ?

Source: Bosch SHX56B02UC Dishwasher