When given the task, the fort is empty, making it impossible to take it. To someone know how to fix this in someway?

Try waiting 32 days. Unless Tullius is having a month-long tea party, then the enemies should respawn in that time.

Please do not post Spoilers in General.

People come to General to learn about the game without learning about what happens in the game. If they want to know about what happens, that is what the Cheats, Hints and Spoilers section is for.

Do not use quest details or location specific details related to quests/puzzles/NPCs in your posts in this section. General is for discussing gameplay and mechanics and general topics about the the game including but not limited to races, items and their use, magic, enchanting, skills, character builds, criticism, praise, funny random things that happen, places from an esthetic standpoint and and other general topics.

Use the Spoilers Section if you have to post quest details or other spoilers or a thread about the same. I will move this now.

Now that the thread is in the proper place, Fort Amol is initially occupied by a bunch of mages who apparently wiped out the Stormcloak garrison. After clearing out the mages, it takes a few days (I don't know how many) before the Stormcloaks reoccupy the place. In that intervening time, the fort is empty, but does not show as "Cleared".

There is a known bug/glitch with the Imperial Legion mission that shows up in a couple of ways. The first way places Stormcloaks in the fort, but no Imperials show up, so you can't rendezvous with your men. The second way does not place Stormcloaks in the fort, so there is no one for your men to fight. As far as I know, both can sometimes be fixed by reloading a save from before you talk to Rikke about the mission. This does not seem to always work, though.

Make sure to check all the buckets in the prison, you will find something nice.

Hint:

I was having the same problem with Fort Sungard. Someone had told me to wait until after 8pm and then talk to whoever is giving you your mission. (Galmar in my case).

Sure enough, it worked. And I had tried doing this quest like 15 times. But the first time I did it after 8pm, it worked.

Try it. Maybe it'll work for you too.