"We go to work at 9 in the morning."
I put the same, and as far as I know, it is correct but as yet not accepted (17/12/26). It's like "We go to work in the morning at 9." vs. "We go to work at 9 in the morning."
As far as I understand, the period of the day (morning, afternoon, evening) always comes before the specific time.
This is because Chinese dates and times are usually expressed from longest to shortest:
e.g.: "去年六月二十三日上午十一点一刻" or "二零零三年十月二日下午六点半"
From what I have experienced putting the time phrase first is more usual for Chinese speaker.
早上 and 上午 both mean 'morning' but 早上 is early morning, 上午 is late morning. Both should be OK, because there is some flexibility about the distinction between the two.