Allow me to provide some troubleshooting steps you can do to fix this error and import your IIF files successfully, @Johnolly2. 

Since this is a data issue, the first thing to do is to repair this data using the Rebuild and Verify Data tools within QuickBooks Desktop. I'll write down the steps so you can perform the task:

1. Go to the File menu, click Utilities, and select Rebuild Data.
2. Click OK on the QuickBooks Information window, then follow the onscreen steps to back up your company data.
3. Allow the tool to repair your file, then once done, select OK.
4. Go to the File menu again, click Utilities, and choose Verify Data.
5. Allow the tool to review and check for any data issues. 
6. If the program didn't detect any problem, select OK. However, if there are issues detected, select Rebuild Now.

Another option you can use is the QuickBooks Tool Hub to fix the issue. 

Furthermore, you can save these resources for more details about managing your IIF files and their formats in QuickBooks Desktop:

• IIF Overview: import kit, sample files, and headers.
• Improved IIF Import in QuickBooks 2019 and later.
• Edit an IIF file.

If you have any other questions or concerns besides importing IIF files, don’t hold back by dropping a comment below. We'll always be here to assist you.