JWT configuration Encoding.UTF8

Question

I would like to implement JWT-based authentication to our new REST API. But var authSigningKey in every response return null, in fact the problem is Encoding.UTF8.GetBytes(_configuration["JWT:Key"])

var authSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(_configuration["JWT:Key"]));

this is my Jwt Appsetting:

"JWT": {
      "Key": "DhftOS5uphK3vmCJQrexST1RsyjZBjXWRgJMFPU4",
      "ValidAudience": "http://localhost:44356",
      "ValidIssuer": "http://localhost:44356"
    }

and it is my code:

private JwtSecurityToken GetToken(List<Claim> authClaims)
        {
            try
            {
                
                var authSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(_configuration["JWT:Key"]));

                var token = new JwtSecurityToken(
                    issuer: _configuration["JWT:ValidIssuer"],
                    audience: _configuration["JWT:ValidAudience"],
                    expires: DateTime.Now.AddHours(3),
                    claims: authClaims,
                    signingCredentials: new SigningCredentials(authSigningKey, SecurityAlgorithms.HmacSha256)
                );

                return token;
            }
            catch (Exception e)
            {
                Console.WriteLine(e);
                throw;
            }
        }

in every encoding it is null.