The statement "If you didn't see it, it doesn't exist" is an example of a logical fallacy known as "argument from ignorance" or "argumentum ad ignorantiam." This fallacy occurs when someone argues that a claim is true (or false) simply because it hasn't been proven false (or true) or because there is a lack of evidence to the contrary.
In the case of the statement you provided, it assumes that the only valid form of evidence is direct personal observation (seeing). It ignores the possibility that there may be other forms of evidence or ways to establish the existence of something beyond personal observation, such as scientific research, testimony from credible sources, or indirect evidence.
In many situations, it's not reasonable or practical to rely solely on personal observation to determine the existence or truth of something. Therefore, dismissing the existence of something solely because it hasn't been personally witnessed is a logical fallacy.