SQL Server and ASP.Net

I receive this message when I try to connect to my database:

Code:

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Login failed for user 'sa'. Reason: Not associated with a trusted SQL Server connection.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Data.SqlClient.SqlException: Login failed for user 'sa'. Reason: Not associated with a trusted SQL Server connection.

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What is wrong? Can't I use "sa" here?