Thanks for joining the thread, @d1csp3r.

This error usually occurs when the file association of the IIF file is changed to other applications than MS Excel. You'll want to verify the type of file you are opening by right-clicking the file and choosing Properties. The file type is listed under Type of File.

If needed, you'll want to change the file association of the IIF file to open with MS Excel. This way, you'll be able to open the file.

To learn more about importing these files, and the maximum number of entries allowed, you may check out these articles:

• IIF Overview: import kit, sample files, and headers

• Import IIF Files

• Maximum number of QuickBooks Desktop list 

• Improved IIF Import in QuickBooks 2019 and later

Please know you can continue to reach me here with any additional questions. Thanks for coming to the Community, wishing you continued success.