Whats a prep file?   Which program are you referring to?  Go here, choose your program and log into your Intuit Account, your downloads should be shown right on the Dashboard

https://www.intuit.com/sign-in/

If you have Lacerte 2018, you can download (and update) Prep Files from within the program

1. Open the current year of the Lacerte Tax program.
2. Click on Settings.
3. Click on Update Preparer Information.
4. Select Download Prep File. 

where is the 2019 prep file

The process is the same for any year (YY). The download should load your 20YY prep file, which will show as Prep.YY into the folder C:\Lacerte\YYtax

For a standalone 2019 it should in C:\Lacerte\19tax as PREP.19

WELL THAT APPROACH LOOKS LOGICAL ENOUGH, BUT THE ROSTER OF AVAILABLE PREP FILE YEARS ENDED WITH 2018, WHICH IS WHAT I WAS USING TO DOWNLOAD THE 2019 PREPFILE?  SORRY FOR THE ALL CAPS. ADVANCING AGE, MEANS DECLINING VISION. 

Have you paid the current year Fast Path Fee?

GO TO "MYACCOUNT''; THERE'S A LINK THERE TO DOWNLOAD *ALL* OF YOUR PREP FILES FOR ALL YEARS.

THEN WHEN INSTALLING THE 2019 (?) PROGRAM, IT WILL ASK ABOUT THE PREP FILE & WHERE IT IS LOCATED.  POINT TO WHERE YOU DOWNLOADED THE PREP FILE.

BUT IT USUALLY ASKS IF YOU JUST WANT LACERTE TO GO ON-LINE TO GET THE PREP FILE.

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MAYBE EXPLAIN A BIT MORE WHAT HAPPENS/GOES WRONG WHEN YOU ARE TRYING TO INSTALL THE PROGRAM.