Formal logic, by the way, means logic where the argument must be true due to its form, without needing to understand its content. E.g. if you know “If X is true, then Y is true.” and you know “X is true” then you can deduce “Y is true”. That argument doesn’t depend on what X and Y are, beyond them being propositions (statements that can be true or false). This is not the kind of argument people actually make when, e.g., debating the merits of the current U.S. president. Some people claim that’s bad and that regular people are arguing wrong, but I think the philosophers are wrong and should learn to understand regular arguments.
A particular informal argument might be wrong, but you can’t refute it just by pointing out that it isn’t deductive.