Why the inventor does not work with the Arduino by connecting to the uart port ,,Note that there is a connection on the Serial USB Terminal application ???

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Serial component works with original Arduino UNO, but it doesn't work with many clones, for example those with CH340/CH341 chip.
It has also worked for me with the CP2102 module.

For other devices, use the extension that Chris has recommended, here are several examples:
https://community.appinventor.mit.edu/t/serial-otg-arduino-ch340-ftdi-rkl099s-extension/21156

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Error 3902 writting the data to serial