why is it impossible to have a sample of methene?

Alkene is a compound with a C=C double bond. In other words, an alkene should contain at least two carbon atoms.

Methene is impossible to exist because it contains one carbon atom only.

Where would the double bond go?

Since alkenes are belongs to the class of unsaturated hydrocarbons at least one double bond is present between the two carbon atoms.

So, alkenes with one carbon atom is not possible.

ene suffix indicates a double bond. H cannot form a double bond
there must be 2 C atoms to form the double bond between them.