Need Help in Asp.Net when i choose Asp.net configuration

Question

Hi All
When I choose Asp.net configuration and choose security to create a Admin Roll an error shows 

"There is a problem with your selected data store. This can be caused by an invalid server name or credentials, or by insufficient permission. It can also be caused by the role manager feature not being enabled. Click the button below to be redirected to a page where you can choose a new data store.

The following message may help in diagnosing the problem: Unable to connect to SQL Server database.

Why this error ecourred Please tell me. I am using visual studio 2005 and sql server 2005 standered version.