If you upgrade your database to NAV2016 using the PowerShell Merge commandlets and have a change to table 49 you will run into this issue.
Table 49 is the Invoice Posting Buffer table that NAV uses to create G/L entries for each unique combination of values in the Sales and Purchase documents. With the NAV2016 release Microsoft have added the Deferral Code field to this table and the primary key.
One of the most popular mods to NAV is to add the description of the lines to this table to inherrit the description of the sales lines to the G/L Entries. This mod is also decribed in my book Microsoft Dynamics NAV 2013 Application Design.
The PowerShell merge tool does not understand that there is a conflict and will just add a second clustered index to the text file which results in this error:
To fix the issue, just remove one of the indexes in the text file and combine them into one.