This error is due to MATLAB is not able to connect with the Hardware. It might be the case that the drivers are not installed properly for the Arduino Uno. Please try to re-install the driver from the Arduino official website.
Additionally, there is an cause related to the background daemon that MATLAB uses to establish connection with the Arduino.
The daemon could be timing out before external mode is able to start successfully. The expected time needed to establish connection varies from computer to computer, and as a result, you may need to modify the timeout value to use external mode. Some users have only noticed this error when I2C sensors are connected to the Arduino.
Please follow these instructions to modify the timeout value: 1. Delete the file, which will be located in the following directory within the Support Package Root Directory: RootDirectory\toolbox\target\supportpackages\arduinobase\+codertarget\+arduinobase\+registry\startDaemon.p
To determine the support package root on your local machine, please execute the following command:
>> SupportPackageRoot = matlabshared.supportpkg.getSupportPackageRoot
For example, if the above command returns "C:\ProgramData\MATLAB\SupportPackages\R2017a\", then the file will be located at: C:\ProgramData\MATLAB\SupportPackages\R2017a\\toolbox\target\supportpackages\arduinobase\+codertarget\+arduinobase\+registry\startDaemon.p
2. Copy the file startDaemon.m (attached file) to SupportPackageRoot\toolbox\target\supportpackages\arduinobase\+codertarget\+arduinobase\+registry\
3. Adjust the timeout value for your system by modifying the "ethernetTimeout" variable at line 80 of startDaemon.m and save the file.
4. Run the following commands at MATLAB command prompt: >> clear classes; >> clear pcode; >> rehash toolboxcache After making these changes, you should be able to run the external mode connection as before. Repeat steps 3 and 4 to tune the timeout value as needed for your system