It is an error to use a section registered as allowDefinition="MachineToApplication" beyond application level. This error can be caused by a virtual directory not being configured as an application in IIS.

Answer:-

I am working .net application. I have this error when i open my application solution file and build my application that time getting this error.

 and then i referred in this link.

he mentioned about I won't open the parent folder solution file. that's the reason happens this error. 

Basically, the error means that there is a web.config file in one of your subfolders that has a configuration element that it should not have. Is this you root/only web config file? If not, could you please post those as well? Also, it sounds stupid, but I would double check that you're opening the website itself in your IDE (and not mistakenly opening a parent folder) I have seen people spend a couple hours trying to debug this same error, when all along they weren't in the right directory. Here is a good explanation on how the web.config hierarchy is set up for ASP that will help you visualize how this works:

http://scottonwriting.net/sowblog/archive/2010/02/17/163375.aspx