Presumably one of R, theta, sign (!), u, y, or c is not scalar but a matrix or column vector. You can't horizontally concatenate scalars with matrices or column vector, hence the error.
Find out which variable is the culprit on why it's not scalar (or why you're using a non scalar variable in your expression).
Also, I strongly recommend changing the name of the sign variable. sign is already a function in matlab which you won't be able to use if you've got a variable of the same name.