You have redefined x to become an array, between the definition of F(x) and the plot.
If you move the plot upper left corner to just above the redefinition of x to an array, your plot should show.
The error message "All evaluations..." is cryptic at first sight. In this case case 'all evaluations resulted in an error' applies, because you cannot compare a singular value (such as -1, 2 or 5 with an array (that you redefined x to).
The definition of F(x) was not the cause of the problem of the OP.
He redefined x to be a 4 element array between the definition of F(x) and the plot (not shown in his picture). That would fail with your definition of F(x) as well.
You're absolutely right. My answer is based on what the photo shows. Only now I have opened the file provided by FedexRight.
Luc already explained why your plot failed - the definition of x as a vector.
Here are a few more suggestions and an explanation, why F(x) fails if x is a vector.
If you are new to this, I understand that the difference between a range and a vector can be quite confusing.
MC15 worksheet attached