Try to use catch in the code snippet to get the exception in the code snippet like below:

import { default as pnp, ItemAddResult } from "sp-pnp-js";
private PnPAddItem():void
{
     pnp.sp.web.lists.getByTitle("Jerry").items.add({
      EmployeeName : document.getElementById('EmployeeName')["value"],
      Experience : document.getElementById('Experience')["value"],
      Location: document.getElementById('Location')["value"]
  })
  .then((iar: ItemAddResult) => {
      console.log(iar);
  })
  .catch((error:any) => {
      console.log("Error: ", error);
  });

}

Try to trace the error in catch for more detailed information and find the cause.

Try to use catch in the code snippet to get the exception in the code snippet like below:

import { default as pnp, ItemAddResult } from "sp-pnp-js";
private PnPAddItem():void
{
     pnp.sp.web.lists.getByTitle("Jerry").items.add({
      EmployeeName : document.getElementById('EmployeeName')["value"],
      Experience : document.getElementById('Experience')["value"],
      Location: document.getElementById('Location')["value"]
  })
  .then((iar: ItemAddResult) => {
      console.log(iar);
  })
  .catch((error:any) => {
      console.log("Error: ", error);
  });

}

Try to trace the error in catch for more detailed information and find the cause.

Update: Try to check the detailed response data using Developer Tool to trace the issue: