FDM Import Error "Error: Import failed.Invalid data or Empty content at FDM Import Script"

In our classical FDM application, I wrote a script to extract data from GL database and import into FDM.

When run the import script, there are data in the "import" screen and there is error message "Error: Import failed.Invalid data or Empty content at FDM Import Script". "Import" fish will not turn orange.

I checked tdataseg table. For those success "import" (fish will turn orange), AccountR, AccountF, AccountX columns will have content. However, for the "import" with error, I notice AccountR, AccountF both show 0. And AccountX does not have content. It seems the mapping process after "import" script did not run.