JWT is not well formed

Again my first time developing an asp.net core rest API. See below snippet:

And below is the error:

"errorMessage": "System.ArgumentException: IDX12709: CanReadToken() returned false. JWT is not well formed: 'System.String'.\nThe token needs to be in JWS or JWE Compact Serialization Format. (JWS): 'EncodedHeader.EndcodedPayload.EncodedSignature'. (JWE): 'EncodedProtectedHeader.EncodedEncryptedKey.EncodedInitializationVector.EncodedCiphertext.EncodedAuthenticationTag'.\r\n at System.IdentityModel.Tokens.Jwt.JwtSecurityTokenHandler.ReadJwtToken(String token)\r\n at System.IdentityModel.Tokens.Jwt.JwtSecurityTokenHandler.ValidateSignature(String token, TokenValidationParameters validationParameters)\r\n at System.IdentityModel.Tokens.Jwt.JwtSecurityTokenHandler.

Below also is my entire TokenManager:

public class TokenManager : ITokenManager
{
//private List listTokens;
private JwtSecurityTokenHandler tokenHandler;
private byte[] secretKey;
public TokenManager()
{
tokenHandler = new JwtSecurityTokenHandler();
secretKey = Encoding.ASCII.GetBytes("xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
//listTokens = new List();
}
public bool Authenticate(string userName, string password)
{
if (!string.IsNullOrEmpty(userName) &&
!string.IsNullOrEmpty(password) &&
userName.ToLower() == "ltcadmin" &&
password == "ltcpassword")
return true;
else
return false;
}
public string NewToken()
{
var tokenDescriptor = new SecurityTokenDescriptor
{
Subject = new ClaimsIdentity(new Claim[] { new Claim(ClaimTypes.Name, "LTC LMM") }),
Expires = DateTime.UtcNow.AddMinutes(5),
SigningCredentials = new SigningCredentials(
new SymmetricSecurityKey(secretKey),
SecurityAlgorithms.HmacSha256Signature)
};
var token = tokenHandler.CreateToken(tokenDescriptor);
var jwtString = tokenHandler.WriteToken(token);
return jwtString;
}
public ClaimsPrincipal VerifyToken(string token)
{
var claims = tokenHandler.ValidateToken(token,
new TokenValidationParameters
{
ValidateIssuerSigningKey = true,
IssuerSigningKey = new SymmetricSecurityKey(secretKey),
ValidateLifetime = true,
ValidateAudience = false,
ValidateIssuer = false,
ClockSkew = TimeSpan.Zero
}, out SecurityToken validatedToken);
return claims;
}

It says that the error is in line 53. Thank you in advance.