smartsnippets otp programming

how to use smartsnippets to OTP？
I use JTAG to download，but the error is
(错误@18-08-10 13:01:40]Adddittional error info at address 0x81FEC (MSB first): FF FF FF FD
(错误@18-08-10 13:01:40]Memory burning failed.
Why？
VPP is 6.8V
VCC is 3.3V