Cannot allocate memory: couldn't create child process

This looks like WordPress is running out of memory on the web server. Checkout the codex link below on how to increase the memory available to WordPress.

https://codex.ww.wp.xz.cn/Editing_wp-config.php#Increasing_memory_allocated_to_PHP

There shouldn’t be, unless you are using XML-RPC to publish posts on your site you may be better off simply deactivating it. The reason it is appearing in your error log may be due to malicious attempts of accessing your site. You can disable XML-RPC by installing and activating the plugin below.

https://ww.wp.xz.cn/plugins/disable-xml-rpc/