If your Galaxy A7 is a (non-Samsung chipset like MediaTek), it might have a real Fastboot mode. Try:
If you need to sideload or use recovery: how to open fastboot mode on samsung galaxy a7 repack
You haven’t installed platform-tools. Download from Google’s developer site and add to PATH. If your Galaxy A7 is a (non-Samsung chipset
This is the most common way to enter the mode if the phone is turned off. how to open fastboot mode on samsung galaxy a7 repack