“不足”here is a noun, It means "shortcoming“”weakness“”imperfection“etc. The general idea of the whole sentence is : 100 points is way too good, if you always get 100, you never have the chance to find out your weakness (you would think that you are already perfect). Now you have 99, congratulations, you are doing fine, but the -1point reminds you what your shortcoming is, You are still able to make a progress.