You must file an amended return to enter the investment sales. Without the entry, the IRS may just assume that the entire proceeds were gains even though they were losses. There is less risk of that these days with cost basis being reported on the 1099-B but you should still amend your return because it is incomplete without the 1099-B entries.
If your return is rejected by the IRS, you can correct the problem and fix the reason for rejection and re-efile your return. If your return is accepted by the IRS, you must prepare, print, sign and mail an amended tax return form 1040X to correct the problem.
DO NOT change your original tax return until you start the amended return. If you have already changed it from the way it was originally filed and accepted, you must change it back before you start the amended return. Wait until you receive the refund from your original return before beginning your amended return to make sure you enter the correct amount. An amended return cannot be efiled. Here is an FAQ to help you:
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
