It's likely that your script isn't located in the same directory as the text file, which is why this error is being raised.
To fix this, try providing an absolute/relative filepath.
Also, when handling files, it's better to use with as it automatically closes the file for you.
So try this:
with open("your_path/sun_og.txt", "r") as f:
file_contents = f.read() ## You only need this line if you're going to use the file contents elsewhere in the code
print(file_contents) ## Otherwise, you could remove the line above and replace this line with print(f.read())
Alternatively, you could use the os module as you can use its chdir function to relocate to another directory (in this case, the directory of your text file). See @Jones1200's answer for more information.