Why am I getting a code 400 from Spotify?

I am trying to get the access token from spotify, but it returns a bad request. I don’t understand what could be causing it.

async function getAccessToken(clientId, code) {
    const verifier = localStorage.getItem("verifier");
    const params = new URLSearchParams();
    params.append("client_id", clientId);
    params.append("grant_type", "authorization_code");
    params.append("code", code);
    params.append("redirect_uri", "http://localhost:5173/callback");
    params.append("code_verifier", verifier);

    const result = await fetch("https://accounts.spotify.com/api/token", {
        method: "POST",
        headers: { "Content-Type": "application/x-www-form-urlencoded" },
        body: params
    });
    
    
    const { access_token } = await result.json();
    return access_token;
}

the clientId, code, and verifier are all there, so I don’t know what is wrong.