"This device uses dm-verity!
This means that swiping to allow system modifications will prevent you from being able to boot if you are using the stock kernel. In order to bypass dm-verity's boot prevention, you will have to install a kernel that has dm-verity disabled in the fstab."
DM-Verity Reference: https://twrp.me/devices/samsunggalaxys6.html
Is most likely the culprit here. What this does is run a check on boot and if you are using the stock kernel and it sees a different recovery or even ROM than DM-Verity expects it will not allow you to boot the OS and only recovery so that you can put the official OS and recovery back. The only way to fix this is to use a custom kernel (or custom ROM that includes a custom kernel),
OR of course if you were to do a full factory image flash (using ODiN you visit Samsung site and get the Factory image meant for your device (be sure model matches your phone's before downloading), put device in download mode and run Odin to flash the factory image. Of course you will lose root and the custom recovery this way, but it should boot.
Since you did not include your actual model number I hesitate to link just any ROM or Kernel as they may be intended for another variety of the S6. If you would like comment or edit your question and add the SM-blah model number and I can be of more help in linking ROMs/Kernels (not really what this site is for, but if you need those to get your phone basically running I can at least find the most popular variety of what you need which should hopefully get you working again. Yuo can google SM-blah ROM XDA and that should give you links to the XDA forums for your device. They are very well know community that does custom Android work of all kinds. I can't say it enough make sure you verify your phone's SM-blah model number against the developers ROM info page to make sure it is actually meant to be used with the exact version of the S6 you have.
Possibly Flashify (DOUBT IT)
Flashify selected the wrong partition automatically (it does warn you to verify it chose the right partition before flash), so you would need to flash a ROM and start again (Flash ROM, GApps AND Recovery.zip [https://twrp.me/devices/samsunggalaxys6.html]) DO NOT FORGET TO FLASH THE RECOVERY IF IT IS IN WRONG PLACE YOU NEED TO GET IT IN RIGHT PLACE BEFORE RESTARTING AFTER FLASHING THE OS IF NOT YOU MAY LOSE THE RECOVERY.