Hello,
In order for you to get better help from the community, it would be helpful if you could specify whether you are using App Engine or Compute Engine and if you are using App Engine since this is the App Engine group, which version of App Engine you deployed your WP to, standard or flex as these have different limitations. Also, was everything working properly before you tried to active this custom wp theme and can you active others wp themes? A screenshot of the exact error you are seeing on your end would be helpful as well. That way we could help you in finding out if the error is due to a mis-configuration of WP or a limitation/issue of/with App Engine.