Hello everyone! I'm a beginner Uno programmer and just was able to figure out a problem. I've worked with a Rex qualis UNO r3 starter kit and had a problem on lesson 6. This lesson involved using a IC called 74HC595 to control 8 LEDs in a line pattern. In my programming joy, i kept clicking upload instead of clicking Verify and then upload. After a long time of getting the same error, I broke down and did the two botton click, verify and upload. Then I found the problem! In my rush to have fun, I had not saved the program correctly. By doing this, everything went back to normal fun.
Takeaway: Make sure to save your project correctly and not in a folder with itself.
Have Fun!