God can be said as an item not having the existence of worldly items
(The existence of Para Brahman is not the existence of non-God items in which the knowledge of the non-God items is a prerequisite condition. Para Brahman exists as per the statement of Veda).
All the non-God items are worldly objects, which are parts of creation. All these items are known first and then only their existence is mentioned. When you say that a pot exists, it means that you are stating the existence since you know the pot already. Hence, the existence of any worldly item requires the knowledge of that item already.
If you do not know anything about an item, you will not say that it exists. Hence, the existence always requires the prior knowledge of the item. But God is beyond world and is unimaginable since God is not known. Hence, the existence of God is not similar to the existence of the worldly items. Since the existence of worldly items, which requires prior knowledge of the item, is absent in the case of God, God can be said as an item not having the existence of worldly items and hence God is non-existent (Asat) in this sense. This does not mean that God is really non-existent because God really exists as per Veda (Astityeva….) and hence God exists (Sat).