You can raid a Heroic stage only when your honor rank has reached level 9.
Here are some general rules of Trial. If you have reached the required honor ranks below for the 4 stages, you can raid the previous finished stages. Otherwise, you need to challenge the previous finished stages that you cannot raid on each day you play Trial. Just go to raise your honor rank.
-> Honor Rank 1 to raid ‘Normal’ stage
-> Honor Rank 4 to raid ‘Elite’ stage
-> Honor Rank 9 to raid ‘Heroic’ stage
-> Honor Rank 15 to raid ‘Immortal’ stage