...
To know more about the different types of error messages and what they mean, look at the following pages:
...
Here is more information on how this can be done:
7.3.8.6 How to setup the creative upload features for Xandr?
7.3.4.11 How to setup the creative upload features for Google Ad Manager?
7.3.4.11.1 How to use your Custom Creative Templates and Native formats for Google AdManager?