One problem appears to be that the function expects a table but you're giving it a categorical array. I can't say for sure because I don't know how you're calling the function, but if you're using the h you define as
h = d.Vertical_Steering_Command % selected a specific variable, categorial (Atltitude, Limbo)
then that's a categorical array (if your comment is accurate).
Another problem is that you're using the variable h to mean two different things inside the function: the input table and the numeric data you want to add to the table.
Try defining the function as follows. I changed the numeric data variable to data (I'm not sure what the output should be so I output the new table h, with the added column).
function h = data_conversion(h)
% convert Vertical Steering Command categorical data to double data type as Table 'h'
% note: Altitude is set to '0', Limbo is set to '6'
data = zeros(size(h.Vertical_Steering_Command)); % error
data(ismember(h.Vertical_Steering_Command,'Limbo'))=6;
% Output double data: add double data type from table 'h' and
% add new column for Vertical Steering Command number data to table 'h'
h.Vertical_Steering_Cmd_num = data;
end
And when you call it, give it a table:
d = data_conversion(d); % d is your table; the function updates the table and returns it, and the new table is stored as d again
