Custom WPF Authentication Problem

I'm creating an application that requires some sort of authentication to ensure data is secured within a SQL database. 

I have managed to find an example of authentication within a WPF application online, shown here; WPF Authentication

However, I have become stuck. I have managed to implement something similar but unfortunately I have come stuck when trying to incorporate a Hash method that calculated the password to convert it to enable a user to login, shown below; 

private string CalculateHash(string clearTextPassword, string salt)
        {
            // Convert the salted password to a byte array
            byte[] saltedHashBytes = Encoding.UTF8.GetBytes(clearTextPassword + salt);
            // Use the hash algorithm to calculate the hash
            HashAlgorithm algorithm = new SHA256Managed();
            byte[] hash = algorithm.ComputeHash(saltedHashBytes);
            // Return the hash as a base64 encoded string to be compared to the stored password
            return Convert.ToBase64String(hash);
        }

My method that authenticates is here;

public UserViewModel AuthenticateUser(string username, string clearTextPassword)
        {
            SchoolAdminDBEntities context = new SchoolAdminDBEntities();

            var userData = context.Users.Where(i => i.Username.Equals(username) && i.Password.Equals(CalculateHash(clearTextPassword, i.Username))).SingleOrDefault();

            if (userData == null)
                throw new UnauthorizedAccessException("Access denied. Please provide some valid credentials.");

            return new UserViewModel(userData.Username, userData.Email, userData.Role);
        }

N.B. My full source code is found here; Source Code

However, I get the following error;

(Credentials saved within database - Username: Mark; Password: Mark ("MB5PYIsbI2YzCUe34Q5ZU2VferIoI4Ttd+ydolWV0OE="))

ERROR:  LINQ to Entities does not recognise the method 'System.String.CalculateHash(System.String, System.String)' method, and this method cannot be translated into a store expression.

Can anyone please help me shine some light in regards to what this means and how I can solve this issue?

Thanks in advance.