Web api not supporting POST method

Change your action to be like Post([FromBody]string schooltypeName) as by default string type is expected to come Uri.

Updated:
Change your body to just "Aided" as currently you would need a class to make the deserialiation work otherwise (ex:class School { public string SchoolTypeName { get; set; } }

See the using namespace at the top of the controller, if you're using System.Web.Mvc, then this problem might be occurred:

Use this:

using System.Web.Http;