OP:
Are you doing a "download from online and install"?
If it doesn't complete, I'd suggest you try another way:
Create a bootable USB installer, and boot from that and install from that.
You'll need:
- a USB flash drive 16gb or larger
- a copy of the Sierra installer (it may currently be in your Applications folder)
- ONE of the following free apps:
-- Boot Buddy
-- DiskMaker X
-- Install Disk Creator
Download one of these.
Initialize the USB flash drive to HFS+ with journaling enabled.
Then use the utility to create the flashdrive.
Then, power down the MacBook.
Plug in the bootable flashdrive.
Press the power on button.
IMMEDIATELY hold down the option key and KEEP HOLDING IT DOWN until the startup manager appears.
Select the USB drive with the pointer and hit return.
You should boot to the installer.
"Point" the installer at your internal drive, and give things another try.
It -might- go easier if you erase the internal drive first, but DON'T DO THIS UNLESS YOU ARE BACKED UP!