Yes. You'll need to amend your 2016 return in order to claim the stock loss. Since it was only a $300 loss, you can safely file the 2017 first without the 2016 stock loss affecting the 2017. Sometimes losses get carried over from one year to the next. Yours isn't that situation. You'll have a few more years actually to file the amendment.
Also, you'll want to file the 2016 1099-B because it's possible the IRS doesn't know it's a loss yet.
Please feel free to post any additional details or questions in the comment section.
https://ttlc.intuit.com/replies/3288565
