I have______seen that video. It was great!
The answer for above sentence is "already" but I doubt why "just" is wrong.
Can anyone explain it to me?

Thank you.

Question

I have______seen that video. It was great!
The answer for above sentence is "already" but I doubt why "just" is wrong.
Can anyone explain it to me?

Thank you.

Answer

'Just' is not wrong. It is fine. 
Several time words could fit in there: Already, just, recently,...

Answer

"Just" and "already" are both correct but may have slightly different meanings.

If you have JUST seen that video, you saw it very recently.

If you say you have ALREADY seen that video you may have seen it recently or a long time ago or any time!