Are you getting mad searching, digging all websites and forums what this error means? Thinking how you can solve the problem? Well now youre in the right direction I will share you how did I solve the problem, I got to the point that I need to share my solutions and trick how did I solve this ePub error, to solve this problem is simple just follow the step by step process.
First Step:
Zipped the mimetype file first, means only mimetype file in a zipd may look similar in the picture below.
Second Step:
Drag and drop the remaining folder to the zipped/mimetype.zip, so now our zipped file contain the entire necessary file and folder for our ePub, may look similar in the picture below.
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Third Step:
Now just rename the zipped file to .epub extensions, mine I just rename it to myeBook.epub see the image below for a sample.
That’s it hope it helps 
, enjoy creating your ePub eBook.