Phone Get stuck with installation Firefox OS on zte a21plus

Hi.
I spent a lot of time to download the code, configure it and build it. So, I hope that someone will help me to get Firefox OS on my phone.
My problem is that when I try to install the system I get an error:
Code:
flashing out/target/product/unagi/system.img failed because the image was too large 
Try re-flashing after running
rm -rf out/target/product/unagi/data && ./build.sh
Of course I've ran this command but it didn't help.
Also I've re-run the commands ./config.sh unagi and ./build. It didn't help too.
p.s.
My phone is ZTE a21plus, know as Unagi. It is rooted and now run with stock recovery and ROM.