I rather disjointed that you cant attack with your shield. A shield is a by far more offensive weapon and that two handed weapons dont get any protection bonuses. Normally you want to keep enemys as far away from your self as possible, range advantage but hey.
Shields add to your Armor Class. Your armor class is a combination of your characters armor, ability to block with a shield, ability to parry with their weapon, and the fact that people don't stand still in a fight.
So if you do not roll high enough to pass someone's AC, your attack fails. The game calls it a 'miss', though it can be due to missing, the enemy dodging, the enemy parrying, or the enemy blocking.
Mechanically, 5E does not differentiate between ANY of that. There's no 'This is your armor AC. Then this is your shield AC. Then this is your AC for moving around. And this one is for parrying'.
So there's no way for the game, or the devs, to decide what's a miss, a dodge, a parry, or a block.
btw you dont want to block at all cost with a shield, this leads to broken hand and by far more damage, you deflect blows with a shield and smack it in a enemys armor gap where it really hurts.
They are also great at additional protection from arrows but still you get a lot of holes on your shield arm.
