I assume that the cell array of numeric 1's shown in the question is the result of the disp(app.UITable.Data(:,15)); line, and that what we see there is the whole column 15.
In that case, it's doesn't make sense to do str2double on app.UITable.Data(:,15) since it already contains numeric data (and not character arrays or strings).
Instead, concatenate the contents of the cells of column 15 together into the numeric vector d and find where d == 1:
d = [app.UITable.Data{:,15}]; %%trovo i "selezionati"
sel = find(d == 1);
Then set the corresponding cells in column 4 to app.Trading(2), converted to a string and wrapped in {} to make it a scalar cell array to match the type of app.UITable.Data, which is also a cell array:
app.UITable.Data(sel,4) = {string(app.Trading(2))}; %gli assegno il valore "from strategy"
