If there is a damage prevention or redirection effect active while assigning combat damage, Arena does not automatically assign damage. You have to do so manually. To do that, you have to use the arrows above and below the creatures/player/planeswalker you could assign damage to. At the beginning, only the arrows above the blocking creatures, which increase assigned damage, are white and active. The arrows below, that reduce assigned damage, are greyed out until at least 1 damage has been assigned. Because you can't assign negative damage, or trample over until lethal damage has been assigned to all blockers. You have to assign exactly as much damage as your creature would deal without damage altering effects to confirm.