You are welcome. @reb77 has written that Samsung doesn’t support fastboot, but I can’t believe that. I will make some investigation regarding fastboot on Samsung
EDIT: As here described, fastboot is available on S7. https://www.tenorshare.com/fix-android/how-to-enter-fastboot-mode-on-samsung.html
Be happy, that the flashing wasn’t working, because with a wrong custom rom, you can brick your device forever 
And sorry, that I have that over read, I have had to warn you asap. Sorry