I submited my run but I got the answer: "Your 22m 21s run in Minecraft: Java Edition - Any% Glitchless - Set Seed, Pre 1.9 has been rejected. Reason: You are required to include the world seed in the submission comment."

What do they mean? Where is the submission comment?

When you submit a run, there is a text box labeled Description. This is where the moderators for that game want to see the seed of the Minecraft world you played on, typed like "Seed: 4929216164286115443".