One edge in the wrong slot
Step 1 / 3
U': Two layers around the top corner: turns counterclockwise (looking at the top corner)
The back center is right. The yellow-blue edge sits in the bottom left slot, where yellow-red belongs.
U' moves the yellow-red edge to the top left. L' drops it into the bottom left slot and pushes yellow-blue out to the front bottom edge. R' moves yellow-blue into the bottom right slot.
Step 1 of 3: U'. Two layers around the top corner: turns counterclockwise (looking at the top corner)