Are you trying to reconstruct a 2016 return that was filed previously by your CPA? You didn't say why you are trying to do all this, which can be tedious and painstaking.
It sounds like you are using Forms Mode to make data entries, which is not recommended. Normally, the Interview Mode (step by step) is the best way to make
entries. There is often a complex relationship between forms and
worksheets in the Forms Mode, as this example illustrates. And overriding entries can cause errors due to the ways that info sometimes flows between forms and worksheets.
In the ideal situation, at least when one is preparing a new return, Forms Mode should be reserved for problem solving, troubleshooting, verifying info flow correctly, minor corrections such as typos, etc.. It is not recommended to use it as the main source of data entry.
You cannot use the "Open Form" icon to open a Form 8949 that way until a Schedule D has been started. As the message indicated when you tried, there must be a Schedule D first.
Go to the interview for the Schedule D for whatever type of capital gains/losses you have. If you have transactions of stocks, mutual funds, bonds, etc., it will be the interview for the 1099-B.
Once you make an entry there in Interview Mode for an investment transaction, that should create a Schedule D, then when you look in Forms Mode again, the Form 8949 may appear automatically in the left-column forms list.
