Garage door immediately reversing when trying to close
Does the door move freely?  If the sensor isn't obstructed, the other cause for reversal would typically be down force.   If it takes too much force to close the door, it may think it's obstructed by something that's not blocking the sensor, and will reverse.   The amount of force required to reverse is adjustable, but you shouldn't adjust it without understanding why it changed, as it is a safety issue if not adjusted properly (think crushing a pet or toddler).    Please consult the instructions for your unit for proper adjustment.