Authorizing access with JWT tokens in ASP.NET MVC website

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C#

ASP.NET

I would like to use jwt tokens for authorization in my ASP.NET MVC website. I have already created an api that generates the jwt token from this tutorial : https://www.c-sharpcorner.com/article/asp-net-web-api-2-creating-and-validating-jwt-json-web-token/

What I have tried:

Now I have added the following nuget packages in my asp.net mvc website :

System.IdentityModel.Tokens.Jwt 5.5.0 
Microsoft.Owin.Security.Jwt 4.0.1
Microsoft.AspNet.WebApi.Owin 5.2.3
Microsoft.Owin.Host.SystemWeb 4.0.1 

And I have also created a startup file and inserted the following code:

app.UseJwtBearerAuthentication(
                 new JwtBearerAuthenticationOptions
                 {
                     AuthenticationMode = AuthenticationMode.Active,
                     TokenValidationParameters = new TokenValidationParameters()
                     {
                         ValidateIssuer = true,
                         ValidateAudience = true,
                         ValidateIssuerSigningKey = true,
                         ValidIssuer = "example.com", //some string, normally web url,
                         ValidAudience = "example.com",
                         IssuerSigningKey = new

  SymmetricSecurityKey(Encoding.UTF8.GetBytes("my_secret_key_12345"))

                     }
                 });

I am then using postman to create a jwt token and challenge the website for authorization. The generate token method is the following. (literally like the tutorial):

public Object GetToken()    
    {    
        string key = "my_secret_key_12345";
        var issuer = "example.com";  //normally this will be your site URL    
      
        var securityKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(key));    
        var credentials = new SigningCredentials(securityKey, SecurityAlgorithms.HmacSha256);    
      
        //Create a List of Claims, Keep claims name short    
        var permClaims = new List<Claim>();    
        permClaims.Add(new Claim(JwtRegisteredClaimNames.Jti, Guid.NewGuid().ToString()));    
        permClaims.Add(new Claim("valid", "1"));    
        permClaims.Add(new Claim("userid", "1"));    
        permClaims.Add(new Claim("name", "bilal"));    
      
        //Create Security Token object by giving required parameters    
        var token = new JwtSecurityToken(issuer, //Issure    
                        issuer,  //Audience    
                        permClaims,    
                        expires: DateTime.Now.AddDays(1),    
                        signingCredentials: credentials);    
        var jwt_token = new JwtSecurityTokenHandler().WriteToken(token);    
        return new { data = jwt_token };    
    }

I have created the following method to challenge the authorization:

[Authorize]
     public string checkbystring()
        {
            return "worked";
        }

But when I test it in postman, the following error keeps popping up: IIS 10.0 Detailed Error - 401.0 - Unauthorized

Any suggestions to make this work would be highly appreciated.