I just opened app designer and tried to set the value of a spinner to 1.3139 + 0.0069i. I received the same error as you.
'Value' must be a double scalar.
This is a poorly written error because 1.3139 + 0.0069i is a double and is a scalar.
class(1.3139 + 0.0069i)
isscalar(1.3139 + 0.0069i)
But it makes sense that the spinner should be a real number.
The solution is to fix the problem with the value of app.B. My guess is that it should not be an imaginary number. Look into how app.B is being computed. If it should be an imaginary number, it's not clear to me how that would be represented in a spinner object.