It can only be called a coup if it's successful in actually changing the government. If it's not, you can argue endlessly (as in the Jan. 6 case) if it qualifies as an "attempted coup" (which implies that it had a remote chance of succeeding).
> if it qualifies as an "attempted coup" (which implies that it had a remote chance of succeeding
No, an "attempted coup" means an attempt at a coup d'état. Probability of success doesn't matter for an attempt to be classified as an attempt, only intentions.
Yes, I think a coup implies that it was perpetrated by people already in government, such as the military, or an opposition party. A riot by the populace that aims to violently remove or influence the government is more properly called an insurrection, a la the French Revolution.
People already part of the government could be involved in triggering an insurrection, such as with the Jacobins involved in the Insurrection of 1792, but it still counts as an insurrection and not a coup because it is carried out by the population, not by forces working for the government, such as assassins or the military.