I have seen several other reports of problems with the Cloud Editor's Serial Monitor on ChromeOS machines:
- Serial Monitor sometimes doesn't do anything on ChromeOS
- Cloud IDE Chromebook Serial Monitor only works about 1 out of every 10 attempts
- Error in web editor serial monitor
- ChromeOS Serial Monitor - Doesn't Work with Uno R4 Wifi
- Serial Monitor on Chromebook
- Serial Monitor on Chromebook - #10 by markgoldner3
- Serial Monitor on Chromebook - #11 by mistermoses
- Uno R4 Minima stopped connecting to the computer
However, I have not experienced any problems using it on my own ChromeOS machine.
Please tell us which USB to serial bridge chip your "Elegoo Uno R3" board has. This is a black chip near the USB socket.
The chip will usually be identified by writing on the top. This might say something like "Atmel MEGA16U2" or "WCH CH340G" or "SILABS CP2102".
Examples:
Microchip ATmega16U2
WCH CH340
If it isn't clear, alternatively you can provide the link to where you bought the board from and we'll see if we can determine the chip from the product listing.