When an argument fails, the failure establishes that the particular inference is unsound. It does not show that every proof is invalid or that inferential knowledge is impossible.
An error may lie in the truth of a premise, the structure of the argument, or the claimed entailment. Correcting the defect can make successful reasoning possible.
The failure of some reasoning is not the failure of reasoning as such. The possibility of inferential knowledge depends on sound premises and valid inference, not universal infallibility in human argument.