need help in Zte Obsidian Z820

handset is not boot properly..

1. Power Off Phone , Remove battery , Insert back
2. Insert USB cable. In some cases require hold BootKey

Wait for phone…
Phone found! [ 4 ]
Sync…
Inital Boot Ok!
BB_CPU_PID : 6735
BB_CPU_NME : [MediaTek] MT6735_S00
BB_CPU_EXT : 0xCA00 , 0x8A00 , 0x0000
Processing BROM stage
Settings for BROM configured!
SecCfgVal : 0x05000000
BromVer : 0x00FF
BLVersion : 0x0001
PreLoader : Active [ Boot with PL ]
BootLdrSS : LOCKED with CUSTOM BL
Processing DA stage
DA Select done, will use MTK_AllInOne_DA_v5.1552.01.01
Sending and initialize DA …

[DA_ERROR] : SECURITY_BOOT_NOT_ACCEPTED
Boot Error!

Operation Failed
Elapsed: 00:00:17
Reconnect Power/Cable!