Let me share some information about importing .iif files. To verify, are you trying to migrate your QuickBooks Desktop (QBDT) file to QuickBooks Online (QBO)? If so, you'll want to follow the detailed steps found in this article to transfer your file: Move your QuickBooks Desktop file to QuickBooks Online.
However, if you're importing .iif files directly into QBO, you'll need to convert them to .CSV file first. You can check on the website for third-party applications that can handle file transfer from .iif to .CSV.
I'll be adding these articles for your reference. You'll also see steps to guide along. Feel free to check.