Hi fmabrouk,
In fact, if you just want to download the code, I highly suggest you use the MCUBootUtility tool instead of the MFG tool, MCUBootutility tool can be downloaded from this link, it is based on the GUI, and more easy to use, just need to provide the app firmware is OK. This tool is also based on the elftosb, blhost and sdphost.
https://github.com/JayHeng/NXP-MCUBootUtility/releases/tag/v3.0.0
the related user manual is:
https://github.com/JayHeng/NXP-MCUBootUtility
But talk back to your MFG tool question, please refer to this application note step by step:
https://www.nxp.com/docs/en/application-note/AN12108.pdf
.S19 is also OK, it is just the .srec, please don't worry.
You also need to use the imx-flexspinor-normal-unsigned.bd, program_flexspinor_image_qspinor.bd, do you also use that file?
I suggest you use the MCUBootUtiltiy tool to download the code at first, after it is success, you can try the MFG method again, the MCUbootUtility also have the detail command log.
BTW, in imx-flexspinor-normal-unsigned.bd you also can set your enterPointAddress to your real app entry point.
Eg, the following .srec or .s19, the entry point is :
entryPointAddress = 0x60004699;

Wish it helps you!
If you still have questions about it, please kindly let me know.
Best Regards,
Kerry