Call to undefined function upload_is_user_over_quota()

In digging around this seems to be a bug with WordPress added in 4.9.8. This patch seems to have broken attachment uploads over the REST API on Multisite, because it uses a function from a file that isn’t loaded for REST requests.

I’ve reported the bug here.

A quick and dirty fix would be to add this to your theme’s functions.php file, or a simple plugin:

if ( ! function_exists( 'upload_is_user_over_quota' ) ) {
    function upload_is_user_over_quota( $echo ) {
        return false;
    }
}

The function won’t do what it’s supposed to do, but it will prevent the fatal error from occurring.