The ^ operator appears to be trying to convert its input into a double precision value, but because a table array can contain any type of data (not just numeric but also text, categorical, etc.) that conversion is not defined.
On the line immediately before you attempt to square TurbulentFluctuation you should extract the contents of the table column using curly braces not the table column itself using parentheses.
A = array2table(magic(5)) % A is a table
st = A(:, 2) % st is also a table
d = A{:, 2} % d is a double array
You can square the elements of d using the .^ (element-wise power) operator [note: not the ^ (matrix power) operator] but you can't square the elements of st. That operator is not defined for table arrays.
y = d.^2 % works
z = st.^2 % error
