IIF for invoice - getting an error that the INVITEM is not a valid header field.

Welcome to the QuickBooks Community. I'll share some information about .IIF files in QuickBooks Desktop and how it works, IFFGus. 

When importing an .IIF file in QuickBooks Desktop, you'll need to make sure that the formats are accurate. An incorrect mapping of the format can be the reason that you'll receive an unexpected error message. That said, I suggest downloading the sample. To learn more about the header placement and columns, consider checking out this article: IIF Overview: import kit, sample files, and headers.

I'm also adding this article that tackles importing and exporting IIF files to and from QBDT for further guidance: Export, import, and edit IIF files.

Lastly, you can perform other tasks with your IIF files, such as bulk editing them, etc. Please head to this article for more insights: Tasks you can do with IIF files.

Feel free to tap me any time for other questions you may have with IIF files. I'm here to answer them for you.

@IFFGus  RE: I build the file by am getting the following error:

; [ERROR] INVITEM is not a valid column name for record INVOICE. [15101]

Did you add INVITEM on the !TRNS row as a field? If so, that will cause the message above. INVITEM should be only included on the !SPL row.

Note you can bypass this sort of overly-restrictive message by importing using the alternate method, which uses the old, original, and much better code to import IIF files. Among other things, it will let you over-specify fields as you may have done, fields which it just ignores instead of stopping with cryptic messaging when there is no need to do that.

For more details, see BRC QuickBooks 2019+ IIF Import Changes and Recommendations