Security damaged, please Repair Security and try again… Sam/2.1.6

hi, I read on many post the people are having this when they tray to fix IMEI or Unlock his phone, I try to do something on many versions and always I get the same result "Security damaged, please Repair Security and try again"… the only thing is going back writing the "first" security backup… if I try something different just I get "Security damaged"

after I do my first try I notice this message on my screen…
Security Error
This phone has been flashed with unauthorized software & is loocked.
Call your mobile operator for additional support.
Please note that repair/return for this issue may have additional cost.

and when I check my IMEI just I get the number 350000000000006/17

the SN still normal

any way to fix this?!?! thanks in advance

all this are happen on my S6 Edge Tmobile Android 5.1.1 G925TUVU3DOJ7
I ear what the only way is downgrade it but is impossible at this time…