Can not find entrance to Korvanjund during the quest "The Jagged Crown"

Solution 1:

I'm confused. You started the civil war quests a while ago and never finished because they were time consuming and boring? The Jagged Crown is the first one after joining the legion.

Anyway, this is a bug, the rubble is supposed to be there when not on this quest and cleared away once the quest started. Either because you waited so long or because one of your mods this did not happen. You can try to either reload a save before coming to Korvanjund and try again or you use the console to bypass the rubble.

MarkForDelete may be able to delete the rubble. Target it, enter the command and reenter the dungeon.

player.moveto 000198BB should teleport you to Rikke.