Ubuntu下pycharm无法导入类的解决方法

最近在学习Python的flask框架，配置好环境后，在终端跑了一下“hello.py”效果不错，一点毛病没有；但当我用pycharm打开后却发现代码有错，无法导入 Flask 类，详细错误信息如下：

This inspection detects names that should resolve but don't. Due to dynamic dispatch and duck typing, this is possible in a limited but useful number of cases. Top-level and class-level items are supported better than instance items.