Longstreet dragged his posterior on the second day and failed to take a wide-open Little Roundtop and potentially roll up the Union line. His tardiness allowed the Yankees to(barely)occupy LRT in time to repel the Rebel assaults.
There were a number of failures on both sides at Gettysburg.
In addition, Lee made a fundamental miscalculation in seeking battle at Gettysburg instead of withdrawing, going around Meade and menacing Washington, or taking defensive positions and goading Meade into the disadvantages of attack.
Arguably, Longstreet and other subordinates may have failed to make the best choices at Gettysburg, but Lee's style of command lacked clarity and permitted too much discretion. This often led to confusion and poor coordination that is most properly blamed on Lee instead of his subordinates.
Finally, where the Union war effort gradually gained in discipline and focus, that of the Confederates was an often brilliant improvisation crippled by state's rights doctrines and the egotism of Confederate political figures and field commanders.
“Longstreet dragged his posterior on the second day...”
There was a YouTube video were they re-created the flank march on the 2nd Day at Gettysburg. The instructions to the cavalry scouts leading the line of march was to avoid observation by the Union scouts assumed to be on Little Roundtop. Bottomline: there was a lot of counter-marching because the topography was not well known and the nearer road was observable from LRT. Longstreet’s assault took hours longer than anticipated to reach their jumping off point for the assault.
Lee's battle plan for July 2 aimed at taking Cemetery Hill and attacking what Lee regarded as weak Union flanks that resulted from the poor disposition weak organization of their forces. Longstreet did not attach great importance to taking Little Round Top because Lee's plan did not.
Why was that? As it was, Lee did not want or expect a bloody positional battle at Gettysburg. He intended on swiftly defeating the federals and then moving deeper into the North and menacing several major cities so as to spur antiwar sentiment. Lee's battle plans at Gettysburg thus aimed at such a result, not a pitched battle.