That's a half-truth, at best. If he did call for unity, he usually undermined it with a disclaimer that pretty much nullified his original call for unity. Not a peep after the cops were killed in Dallas, but he did have people indirectly responsible for those killings (BLM) to the White House, not to mention other extremists.
Obama spent 8 years ignoring the white middle class while wrecking the system that created it; any “calls for unity” were thinly veiled chastisements of whites (bitterly clinging to their guns and Bibles). Trump’s election was a direct result of Obama’s divisiveness.