This value must be a scalar?

Sorry if this seems to be a basic question but I cant figure it out how to correct this.

What I'm trying to do is to test the values of each number in the matrix "a" above if it is less than 0, if it is then I will have the result in "b" which will show only the values less than 0 in matrix "a". I've try the above program but results in the error "This value must be a scalar"

Thank you for your help.