How do I negate a condition in PowerShell?

You almost had it with Not. It should be:

if (-Not (Test-Path C:\Code)) {
    write "it doesn't exist!"
} 

You can also use !: if (!(Test-Path C:\Code)){}

Just for fun, you could also use bitwise exclusive or, though it’s not the most readable/understandable method.

if ((test-path C:\code) -bxor 1) {write "it doesn't exist!"}