Hi makumbi,
Please check the below steps.
- The page method must have the System.Web.Services.WebMethod attribute i.e. [WebMethod]
- The page method must be public.
- The page method must be static.
- The page method must be defined on the page.
- The ScriptManager must have EnablePageMethods set to true.
Refer below article.
If these steps does not work for you then share your code and html.