You can work on them...but whether you can actually file or not depends on what forms your tax return ends up using

AND...the IRS has said they are not going to start processing any until 29 January.  Filing earlier does not mean they are processed first so don't file until you have every tax form in hand, and every scrap of information available.

You can start, and fill in what you have now....then exit and come back tater to finish it.  Just make sure you write down your UserID and Password and keep it safe somewhere...and use the same account every year