They are colloquially interchangeable in American English. We say "In a little bit," "In a little while," "In a little," etc. I reported it as an answer that should be correct.
Please see the sentence and its translation at the top of this page and you'll notice that "It opens in a while." is exactly what it says.
If your sentence wasn't accepted you should have made a Report:
Here's how to report.
Go to the bottom of the exercise page where you'll see REPORT Click on that:
Then choose what you want to REPORT: (You'll see these to choose from.)
-The audio does not sound correct.
-The dictionary hints on hover are wrong or missing.
-The Greek sentence is unnatural or has an error.
-The "Correct solution" is unnatural or has an error.
-My answer should be accepted. This is the one you should click on if your translation was not accepted.
After choosing the REPORT you want to make you should post on the comment page.
Just as I thought. You didn't check the answer shown and assumed it was the use of 'shortly'. The problem was the use of "will" which we didn't have in our database. I've just added it.
And you should use the "Report" option at the bottom of the exercise page then make your comment here. That way we can see your full sentence and be ofmore help.