You put your template in the wrong place. From the Flask docs:
Flask will look for templates in the templates folder. So if your application is a module, this folder is next to that module, if it’s a package it’s actually inside your package: See the docs for more information: http://flask.pocoo.org/docs/quickstart/#rendering-templates
flask只会查找相邻目录里的templates
原文：https://www.cnblogs.com/yjybupt/p/11548281.html