Dropbox no longer offers the option to retrieve a new long-term access token. Instead, it issues short-term access tokens and optional refresh tokens instead of long-term access tokens.

Applications can still gain long-term access by requesting "offline" access, at which point the application receives a "refresh token" and can retrieve new short-term access tokens as needed without further manual user intervention . You can find more information in the OAuth Guide and Authorization Documentation.

The process of retrieving access tokens and optional refresh tokens cannot be fully automated. This requires the user to do it manually at least once. If your application needs to maintain long-term access without requiring the user to repeatedly manually reauthorize, it should request "offline" access in order to obtain a refresh token. Refresh tokens do not expire and can be stored and reused to obtain new short-lived access tokens when needed without requiring the user to manually reauthorize the application.

I found the solution

Step 1: Log in for the first time through the authorization/login URL. After completing the authentication, you will get an access token and a refresh token. Save the refresh token in the database or environment file. It has a long lifespan. (https://github.com/kunalvarma05/dropbox-php-sdk/wiki/Authentication-and-Authorization)

Step 2: Using the refresh token, generate a new access token using the following code

public function refreshToken()
    {
        $arr = [];
        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, 'https://api.dropbox.com/oauth2/token');
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
        curl_setopt($ch, CURLOPT_POST, 1);
        curl_setopt($ch, CURLOPT_POSTFIELDS, "grant_type=refresh_token&refresh_token=<refresh_token_here>");
        curl_setopt($ch, CURLOPT_USERPWD, '<APP_KEY>'. ':' . '<APP_SECRET>');
        $headers = array();
        $headers[] = 'Content-Type: application/x-www-form-urlencoded';
        curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
        $result = curl_exec($ch);
        $result_arr = json_decode($result,true);
        

        if (curl_errno($ch)) {
            $arr = ['status'=>'error','token'=>null];
        }elseif(isset($result_arr['access_token'])){
            $arr = ['status'=>'okay','token'=>$result_arr['access_token']];
        }
        curl_close($ch);
        return $arr;
    }

Call this function to obtain a new access token