这个错误通常表示您正在使用的库或框架不支持HTTP/2，而APNS现在已经迁移到了HTTP/2。您需要确保您正在使用的库或框架支持HTTP/2，或者使用基于HTTP/2的新库。以下是一个解决方法的示例，可以使用基于HTTP/2的curl库来发送APNS通知：

// curl command
curl --http2 -X POST \
  --cert /path/to/cert.pem \
  --key /path/to/key.pem \
  --header "apns-topic: com.your.app.bundle.id" \
  --data '{"aps":{"alert":"Hello World!"}}' \
  https://api.development.push.apple.com/3/device/device_token

// Using curl library in C
#include 

void send_notification(char* device_token, char* payload) {
    CURL* curl;
    CURLcode res;

    curl_global_init(CURL_GLOBAL_DEFAULT);

    curl = curl_easy_init();

    if(curl) {
        struct curl_slist *headers = NULL;
        headers = curl_slist_append(headers, "apns-topic: com.your.app.bundle.id");

        curl_easy_setopt(curl, CURLOPT_URL, "https://api.development.push.apple.com/3/device/device_token");
        curl_easy_setopt(curl, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_2_0);
        curl_easy_setopt(curl, CURLOPT_POST, 1L);
        curl_easy_setopt(curl, CURLOPT_POSTFIELDS, payload);
        curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, strlen(payload));
        curl_easy_setopt(curl, CURLOPT_SSLCERT, "/path/to/cert.pem");
        curl_easy_setopt(curl, CURLOPT_SSLKEY, "/path/to/key.pem");
        curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);

        res = curl_easy_perform(curl);

        if(res != CURLE_OK)
            fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));

        curl_slist_free_all(headers);
        curl_easy_cleanup(curl);
    }

    curl_global_cleanup();
}