My best guess is that you need to update your prep file. YFrom the current year, Setting > Update Preparer Information > Download Prep File > Use 1st radio button to Update My Program Now You will then be able to download all of the prior years in the next screen. Then copy the prep.16 to the x://lacerte/16tax folder.
Good Afternoon Ya & welcome to the Accountants Community!
If you are getting an error that says "Unable to validate your online credentials" I recommend that you run the 2016 Lacerte Websetup, this application is in your Start menu. The link below will give you more detailed steps.
Stay safe and keep in touch!