How to make Post Request using ReactJS – SEO

Hi I am student developer at ReactJS. I am trying to learn how to code with Reactjs Context Api. When I send a data from my form for categories, I am facing an error like this

Unhandled Rejection (TypeError): Failed to execute ‘fetch’ on
‘Window’: Request with GET/HEAD method cannot have body.

What is the meaning of this? What i am mising when I do this process? Fallowing code includes my methods.

my context part :

import React, { Component } from 'react';

const CapsuleContext =  React.createContext();

const reducer = (state,action)=>{
switch (action.type) {
      case "SAVE_CATEGORY_TO_API":
        fetch("http://localhost:3000/categories/", {
          headers: { "content-type": "application/json",
          "Accept":"application/json" },
          body: JSON.stringify(action.payload)
        })
        break;


    default:

    return state
}
}

export class CapsuleProvider extends Component {
    state = {
        categories: [],
        apiUrl: "http://localhost:3000/",
        dispatch : action => {
          this.setState(state => reducer(state,action))
        }

      }

      getCategories = () => {
        fetch(this.state.apiUrl + "categories")
          .then(res => res.json())
          .then(data => this.setState({ categories: data }));
      }



    render() {
        return (
            <CapsuleContext.Provider value = {this.state}>
                {this.props.children}
            </CapsuleContext.Provider>
        );
    }
}
const CapsuleConsumer = CapsuleContext.Consumer;

export default CapsuleConsumer;

my categoryAdd component :

import React, { Component } from 'react';
import { Button, Form, FormGroup, Label, Input, ListGroup, ListGroupItem } from 'reactstrap';
import { Link } from "react-router-dom"
import CapsuleConsumer from '../../context/Context';
import CategoryList from './CategoryList';

class CategoryAdd extends Component {


    handleChange = (event) => {
        let value = event.target.value;
        let name = event.target.name;
        this.setState({ [name]: value })
    }

    handleSubmit = (event) => {
        event.preventDefault();

    }
    saveCategory = (event, dispatch) => {
        dispatch({ type: "SAVE_CATEGORY_TO_API", payload: {id : event.state.id , categoryName : event.state.categoryName , seoUrl:event.state.seoUrl}})
    }



    render() {
        return (
            <CapsuleConsumer>
                {
                    value => {

                        const { categories, dispatch } = value;
                        return (
                            <div>


                                <div>

                                    <Form className="mt-3  font-weight-bold" onSubmit={this.handleSubmit}>
                                        <FormGroup className="text-left">
                                            <Label for="id">Category Id</Label>
                                            <Input type="text"
                                                name="id"
                                                onChange={this.handleChange}
                                                placeholder={categories.length + 1}
                                            />
                                        </FormGroup>
                                        <FormGroup className="text-left">
                                            <Label for="categoryName">Category Name</Label>
                                            <Input type="text"
                                                name="categoryName"
                                                onChange={this.handleChange}
                                                placeholder="enter a category name" />
                                        </FormGroup>
                                        <FormGroup className="text-left">
                                            <Label for="seoUrl">Seo Url</Label>
                                            <Input type="text"
                                                name="seoUrl"
                                                onChange={this.handleChange}
                                                placeholder="enter a seo url" />
                                        </FormGroup>
                                        <Button color="success" onClick={() => this.saveCategory(this, dispatch)} type="submit">Submit</Button>
                                    </Form>
                                </div>                      
                            </div>
                        )
                    }}
            </CapsuleConsumer>

        );
    }
}
export default CategoryAdd;