Select Tools > Processor > ATmega328P (Old Bootloader) and then try again.
You said before you’re using a Nano. Now you say an Uno. Which is it?
OK, no problem. Since you have an Uno, my previous advice to select Tools > Processor > ATmega328P (Old Bootloader) was incorrect (though it won't have caused any harm). You should select Tools > Board > Arduino/Genuino Uno.
Does that solve the problem?
By the way, the Uno and the Nano are actually quite similar due to using the same ATmega328P microcontroller. The main difference is simply that the Nano is in a smaller form factor.
Maybe to try another option from the “programmer” tab ? now it’s the default one - “avrisp mkll” (honestly no idea what this option (the programmer tab) even do, but just trying to suggest things to do)
The Tools > Programmer menu selection only matters when you do a Tools > Burn Bootloader or Sketch > Upload Using Programmer. Any other time it’s completely ignored and it doesn’t matter what you have selected from that menu. Since you’re doing a regular upload, that has nothing to do with your problem.
Remove any connections to pins 0 and 1. These pins are used for communication with your computer, including uploads. Connecting anything to these pins can interfere with uploads.
Make sure you have the correct board selected from the Tools > Board menu.
Make sure you have selected the port of your Arduino board from the Tools > Port menu.
Sometimes the port will be labeled with the board name in the menu. Other times it will not. If you don’t know which port is your Arduino, you can find it like this:
- Unplug your Arduino board from the computer.
- Tools > Port
- Note the ports, if any, listed in the menu.
- Close the Tools menu
- Plug your Arduino board into the computer.
- Tools > Port - The new port listed in the menu is your Arduino board.
I think we've come to the last resort of burning the bootloader on the problematic Uno. This does require an ISP programmer. If you don't own one, you can use your working Uno as an "Arduino as ISP". You can find instructions for this here:
If you have any questions or problems with that process, let us know and I'll try to help out.