Yes the creature can attack the caster.
Bigby's Hand: You create a Large hand of shimmering, translucent force in an unoccupied space that you can see within range... It has a Strength of 26 (+8) and a Dexterity of 10 (+0). The hand doesn't fill its space. When you cast the spell and as a bonus action on your subsequent turns, you can move the hand up to 60 feet and then cause one of the following effects with it.
Interposing Hand: The hand interposes itself between you and a
creature you choose until you give the hand a different command. The
hand moves to stay between you and the target, providing you with half
cover against the target. The target can't move through the hand's
space if its Strength score is less than or equal to the hand's
Strength score. If its Strength score is higher than the hand's
Strength score, the target can move toward you through the hand's
space, but that space is difficult terrain for the target.
The creature can attack the caster with a melee attack if the creature has more than 26 Strength therefore can move through the hand's space (difficult terrain for the creature) to get in 5ft (10 with a reach weapon) of the caster. The hand will still provide the half cover giving +2 bonus to caster's AC. If the attacker can attack from farther away however the hand can only grant the cover bonus. The reach has to be greater than 10ft to avoid the hand as it's a Large hand and covers 10x10ft square
If the caster would attack with a melee weapon however, the creature can't take advantage of the half cover as the hand specifies one way advantage (bold for emphasis on description).
Also any creature not targeted by the interposing hand will have no trouble getting close or attacking the caster.