Checking the server can send notifications

Run "php modules/addons/whmcspwa/tools/selftest.php" on the server. It signs a token, signs a VAPID request and encrypts then decrypts a real push payload, without touching your database. Eighteen checks should pass. If the push ones fail, the server is missing the OpenSSL support that encrypted notifications need, and no amount of configuration will fix it: ask your host to enable openssl_pkey_derive and AES-128-GCM.