Honor 4C (Marshmallow) Doesn't Connect to PC In Fastboot and Rescue Mode
You need fastboot installed in order to use fastboot commands.

Download Minimal ADB and Fastboot which is a command-line tool. HiSuite doesn't connect to devices via fastboot.

Install it and run it, then type fastboot devices to check if the device is recognised.

To proceed with the unlock process, type fastboot oem unlock, this should prompt for a password then enter the password that was given to you. 
Or simply type fastboot oem unlock xxxxxxxxxxx. Replace the code received from Huawei.