Data type of dropdown.Value is a character, so you need to covert it to number.
The following will work. Please replace "plot" command depending on your data.
plot(app.UIAxes, 1:20, 1:20)
app.xl = xline(app.UIAxes, str2double(app.DCValueDropDown.Value));
function DCValueDropDownValueChanged(app, event)
value = app.DCValueDropDown.Value;
app.xl.Value = str2double(value);
Below is a preview.