If you dont know if something cannot be you cannot claim that it can be. You can only claim that it is possible that it can be or as I originally phrased it that it is possible that it is possible.
Something is either possible or impossible. That's the Law of Excluded Middle. A Law of Thought. Fundamental Classic Logic.
If something is not impossible, then it is possible. Logic.
Nothing changes if you add the epistemic dimension (knowing).
If you know that something is not impossible, then you know that it is possible. Logic.
You appear to be resting your argument on epistemic ignorance: If you don't know whether something is possible or impossible, then....
But this doesn't change the fact that something is either possible or impossible.
If you don't know whether it is possible or impossible, you cannot assert anything either way -- you cannot say that it is possible and you cannot say that it is impossible.
You want to say that if you don't know that it is impossible, then it only "can be" possible.
But your "can be" is an expression of possibility. You want to say that ignorance of impossibility entails knowledge of possibility. But you cannot say this.
If you don't know that it is impossible, unless you know that it is possible, you cannot say that it "can be" possible.
You are back at the beginning: something is either possible or impossible.
And this is a necessity, not a possibility. Not a "can be."
In fact, there is no logical contradiction in the concept of an almighty God, and so it is not impossible. And this is known.
Therefor, you know that an almighty God is possible.