AVR upload errors

I have tried using arduino unos to program for an engineering project but for 4 boards i have gotten either the error avrdude: stk500_getsync(): not in sync: resp=0xe0 or avrdude: verification error; content mismatch. i have no idea why these errors come up. I am confident i didnt burn out 4 different boards and since i was uploading the same sketches from when it worked to when it didnt work i dont see why these errors are coming up. does anyone have a legitimate solution to solve either of these two upload errors.

Do you have the correct board selected on the Tools menu?