"This value must be a scalar"

We don't see the definition of S.ts_a.

The variables you show seem to be 1 x 2 matrices. I am not sure if you have created them that way on purpose or accidentially have used the square brackets.

If your variables are matrices consisting of two values on purpose, you will have to decide which of those two values you want to compare against S.ts_a (I suppose this will be a scalar).

For further questions please additionally attach your worksheet.