ArduCopter 4.4.0

Hi all,
I am new to arducopter and trying to build a custom firmware using ArduCopter-4.4.0 . But the complier is throwing error which is shown below. Can anyone help me to resolve this issue?

Do not do that!!
Use at least ArduCopter 4.4.4

I understand, I should use the newest version. But could you help me to understand the problem here to solve this first? Why the compiler is throwing error, where did I made mistake?