I suggest checking the end date of the period you're trying to file.
If the end date hasn't elapsed yet, you'll be unable to submit that specific VAT return. Say, if your end date is 29/2/2020, you can submit it on 1/3/2020.
In case you need to view a list of supported features for MTD, take a look at this article: Supported Features for Making Tax Digital (MTD) in QuickBooks Online and Desktop. It has a table with a list of features that indicates if it's available in QuickBooks Online.
Have other questions in mind? Feel free to drop them in the comments below, and I'll be sure to get back to you.