저는 사무실에서 아직 있어요. It's wrong.

Since you use the verd 있다 you must use the particle - 에 (location, place).

저는 사무실에 아직 있어요. It's correct.

wow, i got great comments here, thank you for all my friends,  i will learn it as well. ^^ Anyeong~~~~

Marta's opinion is right.

When just indicating a location or place, particle '에' is used. "사무실에"

When describing a place or location where a certain action happen, particle '에서' is used. "사무실에서"

나는 사무실에 있어요. O (I'm in/at the office.)

나는 아직 사무실에 있어요. O (I'm still at the office)

나는 사무실에서 있어요. X

나는 사무실에서 일하고 있어요 O (I'm working at/in the office)

"사무실" is "office" in Korean.

I'm still at the office. - 저는 사무실에서 아직 있어요.

I still work at the office. - 저는 사무실에서 아직 일해요.

Oops! Sorry. I was supposed to correct, but I'm the one who got corrected! Haha!