Json Web Token ArgumentOutOfRangeException and Custom Json Web Token Authorization Filter

public HttpResponseMessage Get()
        {
            var secretKey = Convert.ToBase64String(Encoding.UTF8.GetBytes("MYSECRETKEY"));
            var symmetricKey = Convert.FromBase64String(secretKey);
            var tokenHandler = new JwtSecurityTokenHandler();

            var now = DateTime.UtcNow;
            var tokenDescriptor = new SecurityTokenDescriptor
            {
                Subject = new ClaimsIdentity(new[]
                        {
                        new Claim(ClaimTypes.Name, "John")
                    }),

                Expires = now.AddMinutes(Convert.ToInt32(5)),

                SigningCredentials = new SigningCredentials(new SymmetricSecurityKey(symmetricKey), SecurityAlgorithms.HmacSha256Signature)
            };

            var stoken = tokenHandler.CreateToken(tokenDescriptor);
            var token = tokenHandler.WriteToken(stoken);

            return Request.CreateResponse(HttpStatusCode.OK, token);
        }

I'm testing Json Web Token on Asp.Net Web Api but I received error when var stoken = tokenHandler.CreateToken(tokenDescriptor);is executed, the error message that I received was:

System.ArgumentOutOfRangeException
  HResult=0x80131502
  Message=IDX10603: Decryption failed. Keys tried: '[PII is hidden]'.
Exceptions caught:
 '[PII is hidden]'.
token: '[PII is hidden]'
Parameter name: KeySize
  Source=Microsoft.IdentityModel.Tokens
  StackTrace:
   at Microsoft.IdentityModel.Tokens.SymmetricSignatureProvider..ctor(SecurityKey key, String algorithm, Boolean willCreateSignatures)
   at Microsoft.IdentityModel.Tokens.CryptoProviderFactory.CreateSignatureProvider(SecurityKey key, String algorithm, Boolean willCreateSignatures)
   at Microsoft.IdentityModel.Tokens.CryptoProviderFactory.CreateForSigning(SecurityKey key, String algorithm)
   at Microsoft.IdentityModel.JsonWebTokens.JwtTokenUtilities.CreateEncodedSignature(String input, SigningCredentials signingCredentials)
   at System.IdentityModel.Tokens.Jwt.JwtSecurityTokenHandler.CreateJwtSecurityTokenPrivate(String issuer, String audience, ClaimsIdentity subject, Nullable`1 notBefore, Nullable`1 expires, Nullable`1 issuedAt, SigningCredentials signingCredentials, EncryptingCredentials encryptingCredentials)
   at System.IdentityModel.Tokens.Jwt.JwtSecurityTokenHandler.CreateToken(SecurityTokenDescriptor tokenDescriptor)
   at Life_Saving_Society_Malaysia.Controllers.LoginController.Get() in C:\Users\Lucas PC\source\repos\Life Saving Society Malaysia\Life Saving Society Malaysia\Controllers\LoginController.cs:line 35
   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>c__DisplayClass6_1.<GetExecutor>b__3(Object instance, Object[] methodParameters)
   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.Execute(Object instance, Object[] arguments)
   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ExecuteAsync(HttpControllerContext controllerContext, IDictionary`2 arguments, CancellationToken cancellationToken)

Also how do you create a Custom AuthorizeFilterAttribute (Authorize using Json Web Token) for Web API? (NOT Asp.Net Core)