Why can I not use External Mode with my Arduino Mega 2560?

I have an Arduino Mega 2560 connected to my computer and I am using simulink to program it. I can program it fine when I do normal mode, the program works and the Arduino reacts the way its supposed to. However, when I do it in External mode, I get these two errors:

Error occurred while executing External Mode MEX-file 'ext_comm':
Failed to connect to the target. A time-out occurred while waiting for the connection response from the target. Possible reasons for the time-out:
a) The target is not switched on.
b) The target is not connected to your host machine.
c) The application for the model is not running on the target. You might have clicked the Stop button. If the Run button is not dimmed, click it. Otherwise, click the Build button, which downloads and runs your application on the target.
Component:Simulink | Category:Block diagram error
Unable to connect to the 'Arduino Mega 2560' target for 'Lab_2_pt1'.

(btw the file I am using is Lab_2_pt1) Is anyone familiar with this error? Are there any checks I should do?

Also I should mention that I get no errors when I build it even in External mode. The error comes up when I want to run the program.

Answer 1

[Ning Zhang]

Could be resulted by multiple issues

Check out this solution: https://www.mathworks.com/matlabcentral/answers/345567-why-am-i-unable-to-connect-to-arduino-in-external-mode

[abdolreza azizi]

did you fix it? if you do that plz mail me: [e-mail]