From the error message, you've plotted some datetime data that includes time zone information. If you do that, and you want to use line, xline, or yline you need to specify a datetime that includes time zone information in that *line call. Let's make two datetime objects, one with a time zone and one without.
withTZ = datetime('today', 'TimeZone', 'America/New_York')
withoutTZ = datetime('today')
Now let's make some data to plot.
v = 0:5;
x = withTZ + days(v);
plot(x, v.^2);
If we were to try to make an xline using withoutTZ, it would fail. But if we make one using withTZ, it works.
xline(withTZ + days(2.5)); % Works
xline(withoutTZ + days(1.5)); % Fails
