The annoying part about Double Click has been that it is such a moving target, so I recommend following their support guides (and following the spirit of my implementation) and seeing if you continue to receive errors. Their guide starts here: HTML5 banner creative overview - Studio Help (and this url will likely be broken within the month 
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If that continues to not work, can you let me know the error you see in your ad? And perhaps a code sample? It is entirely possible that my guides need updating even though I did my best to future proof the guide I have prepared. Are you working off of this one?