not yet, haven't got round to it

I cannot reply.

私は返事ができません

I still cannot reply.

私はまだ返事ができません

I still cannot reply because work is so busy.

私はまだ仕事が忙しくて返事ができません

I still could not reply yesterday because work is so busy.

私はまだ仕事が忙しくて昨日返事ができません

I still could not reply in the office yesterday because work is so busy.

私はまだ仕事が忙しくて昨日事務所で返事ができません