Mac Sierra - Not able to start in safe mode
Press Power on

After your hear the boot chime...

HOLD Shift key till it is in the Safe mode.

If it is not working lets try other start up modes like Verbose 

Turn on your Mac, then immediately press and hold Command-V. 

or Single user mode Turn on your Mac, then immediately press and hold Command-R

Still nothing ! Reset your SMC and NVRAM.

Rare situation, your EFI portion on the SSD is Brocken. To fix it Back up your data and reinstall OSX.