Welcome.

You need to read the documentation in detail, it shows an example of a request body for PUT method:

{
    "name":"test1",
    "salary":"1123",
    "age":"23"
}

There is no employee_ in the names of the attributes.

So an example flow could look like this:

1) create an employee with POST method:

URL: /api/v1/create

req body:

{
    "name": "DorisWan",
    "salary": "50000",
    "age": "25"
}

res body:

{   
    "status":"success",
    "data":{
       "name":"DorisWan",
       "salary":"50000",
       "age":"25",
       "id":47
    }
}

2) update the employee with PUT method:

URL: /api/v1/update/47 See how I took the id from the previous response body and put it into the URL.

req body:

{
    "name": "Doris",
    "salary": "60000",
    "age": "26"
}

res body:

{   
    "status":"success",
    "data":{
       "employee_name":"Doris",
       "employee_salary":"60000",
       "employee_age":"26",
       "id":47
    }
}

If you wanted that to not error you would do something like this instead in your models.

public class ConfigurationConstant
{
    public string Key { get; set; }
    public string Value { get; set; }
}
public class ConfigurationConstants
{
    public List<ConfigurationConstant> configurationConstants { get; set; }
}

Then in your controller

[HttpPost]
[Route("save")]
public async Task<IActionResult> SaveConfigurationConstants([FromBody] ConfigurationConstants configurationConstants)

firstname, firstName, and FirstName are all different identifiers. They need to match.

Note that following Java conventions will make your life much easier: The names of items that are not classes (variables, fields, methods) should start with lowercase letters. Additionally, following REST conventions will make your life easier: POST already means "add", so don't have an extra /add in the URL, just POST /user.