In Sage 500 knowledge we have this issue logged under #73309. Here is a possible resolution:

The next available line on the Detail tab does not have the same Key value as the Main tab currently being processed. Note: Key values are case sensitive.
For example, when importing Accounts Payable vouchers the Main and Detail worksheets have two MainKey fields: Type and Voucher.
If Mainkey Type field = Invoice on the Main worksheet, but Mainkey Type field = invoice on the Detail worksheet, the user will receive the message "No detail records attached to this row".

If a Main and Detail worksheet exists, the record to be imported must be in the exact same MainKey order on the Main and Detail tabs for DataPorter to recognize them. The values are case sensitive and should not contain extra spaces before and after the values.

Could the version of Excel I'm using be causing an issue - I have the newest version Office 365

Microsoft Office 365 is supported with Sage 500 version 2016 only.