score:1

map is not a method of an object. you can map over its keys using object.keys.

render() {
   const grouplist = object.keys(this.state.post).map((key) => {
       return  (
           <div key={key}>
              <div>{this.state.post[key]}</div>
           </div>
       )
   });

   return (
     <div>{grouplist}</div>
   )
}

however, there are other problems once you fix that but you should try to solve them yourself and ask other questions if you can't

score:2

this is how you map it. just change post with this.state.post

const post = {
  store1: [
    { id: '0001', business: 'ministop' }
  ],
  store2: [
    { id: '0002', business: 'grocery store' }
  ],
  store3: [
    { id: '0003', business: 'seven eleven' }
  ],
  store4: [
    { id: '0004', business: 'big store' },
    { id: '0005', business: 'medium store' }
  ]
};

console.log(object.keys(post).reduce((acccumilator, iterator) => {
  return [...acccumilator, ...post[iterator]];
}, []));

/*
  object.keys(this.state.post).reduce((acccumilator, iterator) => {
    return [...acccumilator, ...post[iterator]];
  }, []).map(data => {
    return  (
           <div key={data.id}>
              <div>{data.business}</div>
           </div>
       )
  })
*/


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