What is this fallacy? Fact abusing with amount of unknown

My friend put out a claim and I refused it with a fact. And he said: ok but the things which you don't know are more than you know. How can be sure my claim is not true? You should say it might be.

yeah he made equal the possibility of claim and a fact, and abused the fact with unknown fantastic things.

#example:
A) Plants also might be suffering when we damaged them. Plus, they have stress also biologicly.
B) Yeah they have stress but with other ways. Not related to nerve system. They dont have nerves.
A) Ok but the things which you don't know are more than you know. How can be sure my claim is not true? You should say "it might be" at least.

is it a combo or one fallacy? lets think.

Let people have their modicum of doubt. It is good form to refuse to make claims of certainty, unless actually certain. For example, 1+1=2, a bachelor can't be married, a circle has no corners, etc. As rationalists, let's acknowledge that we don't operate on certainties (outside of math and logic); we operate on probabilities . Are we certain that plants don't feel pain? I wouldn't want to make that claim understanding that perhaps nerves aren't the only source of pain in all living organisms. Is it likely that plants feel pain? Not at all (I could go on a evolutionary rant as to why, but that would be derailing your question).

In short, I don't see any fallacy here if your friend is simply asserting that you cannot claim that it is impossible (you would need evidence for that), just like you would be right to demand evidence if your friend claimed that it is possible . Then remind your friend that uncertainty does not mean possibility. The former is a epistemic claim, the latter is an ontological one. Meaning that your ignorance (not knowing) does not prove that something is possible.