That's why you will never be able to decrypt your sd card because the key that was used to encrypt your sd card is lost. I hope you have a backup of the sd card data somewhere else.. if you don't, sorry for your loss.
I don't know what steps you took to flash your phone. There's an "official" way and an "unofficial" way. The official way is that you download the original software for your phone, then flash the software to your phone. You either chose a csc or home csc file. The csc file will erase the data on your phone, referred to as clean installation, and the home-csc file will keep all data and simply update the system. You will be able to decrypt the sd card.
The other way requires you to unlock the bootloader which trip knox, reducing the security of your device. Warranty is void. You lose access to secure features like secure folder, health, pay, etc... your data is lost. Then you install a custom recovery to install a custom rom, root the device etc...you won't be able to decrypt the sd card.
*A custom rom is useless nowadays . The stock software comes with all the features and customization you need.
It's important to backup all data before attempting any changes to the device and device software.
Try to restore your data it will request you enter your security code, then your back up may start being decrypted and restored, at that's how it's done on smart switch
