For starters you could either download the manual and find out what Error 21 refers to or take it back to the supplying dealer (Halfords?) or call them.
Or YouTube might just have the answer:
Also that thread refers to older bikes. On the OP's the controller is not in the base of the battery holder, but in a little box built into the underside of the frame just forward of the bottom bracket.
The torque sensors themselves are fine, but can grunge up, as can the slots in the inside of the chainset that the sensor 'sees'. Also the sensor itself is in a bit of a vulnerable position and its not unknown for people to damage them without realising.
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