I was thinking that when watching Rogue One I did not hear or see any mention of Admiral Ackbar during the film. On further investigation I found no mention of Scarif on his Wookieepedia page or of him on the Battle of Scarif page. So where was Admiral Ackbar while the Battle of Scarif was raging?
We don't know where he physically was, but he was apparently unable to adequately provide support in the battle.
Accordining to Star Wars: The Rebel Files, his forces were "Shorthanded, behind schedule, underequipped, and hardly in a position to supplant Raddus and bolster the rebel fleet."
Ackbar was in the original script for the 2016 film Rogue One: A Star Wars Story but the part was ultimately changed to Admiral Raddus after Ackbar appeared in the 2015 film Star Wars: Episode VII The Force Awakens