Your HTML5 creative does not specify an image size (or the size is 0); therefore it cannot be rendered in Ad Manager without some modifications first. Ideally, you should contact your advertiser and request that they submit only HTML5 creatives that meet Ad Manager’s requirements. However, if you need to make the modifications on your own, do the following.
For GWD creatives
Go to the main HTML file included in the bundle. Modify the last part of it to change the minWidth and minHeight variables so that they represent the size of the creative (maxWidth and maxHeight can be ignored).
For non-GWD creatives
If your creative was not produced in GWD (Google Web Designer), then you should update the creative’s meta tags to represent the size (as mentioned in the Size meta tag example).
For example, for fixed size 300x250, you would include the following tag:
<meta name="ad.size" content="width=300,height=250">;
