Hi Daniel L,
This exception occurs because there are several additional features in the Standard Edition which are not defined in the old web.config. Please start from the beginning once again but this time follow the steps bellow:
1. Create new project with Sitefinity 3.1 Standard;
2. Copy the files and folders from the Community web site (without bin
folder) and paste them in the new project (do not replace the existing files);
3. Copy all changes made to the web.config (if any) to the web.config of the new project.
Hope the provided information is helpful.
the Telerik team