What is a Runtime Error and Fixing Runtime Error in ASP.Net

Dear Programmer,

When i am upload my reportviewer (.rdlc) file on server than it gives error message (configuration error), but when i am change my web.config file in my file manager than site doing work but my form data is saved in my database and give runtime error.

how can i fix it, and why this error comes on server, while local does work perfect.