DRK Verify alway Failed

i tried with s6, s6 edge, note 5. all of them get same result. any idea.

Port : COM21 : USB Serial Port (COM21)
Baud Rate : 115200 (Auto)
Device : Samsung
Operation : Device Root Key Fix
Opening Port… OK
Checking For Phone… OK
Entering Test Mode… OK
Reading Phone Information… OK
Model : SM-N920T
Cust Code : TMB
SW Ver. : N920TUVU2COJ5
Unique No. : 0715F742163xxxxxx
IMEI(1) : 353xxxxxxxxxxxx
IMEI(2) : 353xxxxxxxxxxxx
MSL Authorization Skipped
Fixing Device Root Key…. Done
Verifying Device Root Key… Failed
Port Closed.
Operation Failed!
Elapsed Time: 00:10
Module Ver. [1.8]