Thanks for the additional information; it's now clear what's going on.  IDLE has two kinds of windows where code can be entered.  When you launch IDLE, by default it opens with its shell window.  This window is similar to running the Python interpreter in interactive mode, i.e. you can type in lines of Python code that are immediately executed and whose results are displayed.  The other kind of window is an IDLE editor window in which you can edit a file containing Python code (either a new file or by opening an existing file).  When you are ready to run the code in the selected editor window (there may be multiple ones open), you either use the Run menu command or its accelerator shortcut, F5.  The results of running the file are also displayed in the IDLE shell window.

What seems to be happening here is that, if are you in the shell window and enter some text and press F5 rather than return/enter to complete the line, IDLE mistakenly treats the shell window as if it were an editor window and prompts you to save and then tries to run the contents of the shell window.  It shouldn't let you do that.  Thanks for bringing that error to our attention!