Json – I am requesting access token from FHIR sandbox and receive token but error of invalid_grant

I am requesting an access token from FHIR sandbox for a standalone web app (no client secret). On my redirect page I collect code and state from URL, and then fetch access token.

export default function Redirect() {
  // const router = useRouter();

  const searchParams = useSearchParams();

  useEffect(() => {
    const code = searchParams.get("code");
    const state = searchParams.get("state");

    const clientId = "ea9b08eb-030c-41e5-b24b-e4b95ce068e5";
    const redirectUri = "http://localhost:3000/redirect";

    if (code) {
      console.log("code: ", code);
    }

    if (state) {
      console.log("state: ", state);
    }

    if (code && state) {
      const body = `grant_type=${encodeURIComponent(
        "authorization_code"
      )}&code=${encodeURIComponent(code)}&redirect_uri=${encodeURIComponent(
        redirectUri
      )}&client_id=${encodeURIComponent(clientId)}`;

      fetch("https://fhir.epic.com/interconnect-fhir-oauth/oauth2/token", {
        method: "POST",
        headers: {
          "Content-Type": "application/x-www-form-urlencoded",
        },
        body: body,
      })
        .then((response) => {
          response.json().then((data) => {
            if (!response.ok) {
              console.error("Response Error Data: ", data);
              throw new Error(`HTTP error! Status: ${response.status}`);
            }
            console.log("Access Token Data: ", data);
          });
        })
        .catch((error) => {
          console.error("Error:", error);
        });
    }
  }, [searchParams]);

I get access token in console.log, but error of invalid_grant

I was expecting access token which I receive…

{
    "access_token": "eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJhdWQiOiJ1cm46b2lkOmZoaXIiLCJjbGllbnRfaWQiOiJlYTliMDhlYi0wMzBjLTQxZTUtYjI0Yi1lNGI5NWNlMDY4ZTUiLCJlcGljLmVjaSI6InVybjplcGljOk9wZW4uRXBpYy1jdXJyZW50IiwiZXBpYy5tZXRhZGF0YSI6InRiTjlBMk1iT090OXlaZXM5S2ZZY3ZfdFB4XzFNRWQwaktTRlU4YXhnUEdrX2owRzVvV0xibmlOWVg0TFNHT090YnN4OENUVl95d1NSeDhQckNyd2Z0WlZvb0d6cEhIWElfTVU5LUl2bEdoNldzNnhYVXE3RmFIZEEwSXVodDdmIiwiZXBpYy50b2tlbnR5cGUiOiJhY2Nlc3MiLCJleHAiOjE3MjAwNDE2NDcsImlhdCI6MTcyMDAzODA0NywiaXNzIjoidXJuOm9pZDpmaGlyIiwianRpIjoiMmY0M2U2NDItMTc0OS00YjM4LWJlOGQtMjYxOGY4OGMwODgxIiwibmJmIjoxNzIwMDM4MDQ3LCJzdWIiOiJlM01CWENPbWNvTEtsN2F5TEQ1MUFXQTMifQ.OLeMi-B9QWR30fMLHJlZblEUKKYZe5UEnUgvD98Gmzq2B50m1Fofpz_ecVoS0sIGba75eFLw2JX6frEbhN1IPnQAybG0PV8aT1gM7RUSmERVRaw_ZOKJ3e6m8QtiBVbD8yjUH_UIRcMBhXJi3XHNf0KS0YqC1NcYFV8zfJ_xaUvYf4XrgUVAg-5CTNeElocYJ5cH67gn6TMxFOyxSgNEqBECNX-cMAHKAsSTLeUZLaiTOxOHLrWArSW__OuuxK5FGrlS6tHP0qKz1qX2SZUGnfoYDlR5BwDkx87W425aK_15r_7ZcMhxXCsom69gJvp46qAo8X8a1VYVP6ANKOetpg",
    "token_type": "Bearer",
    "expires_in": 3600,
    "scope": "system/DynamicClient.register user/Binary.read user/CarePlan.read user/CareTeam.read user/Communication.read user/Condition.read",
    "state": "abc123"
}

But I also get…

{
    "error": "invalid_grant",
    "error_description": null
}

Can somebody please hep me understand?