It is achiral because it has a plane of symmetry, the plane defined by the atoms of the molecule. In an alkene, the double bonded carbons and the atoms directly attached to them lie in a single plane. If you draw the molecule on paper, this would be the same as the plane of the paper.
Another, albeit weaker, argument you could use is that the molecule contains no stereocenters, that is, it does not contain any carbon atoms with four different groups attached. It is a weaker argument because chirality has fundamentally to do with symmetry and there are molecules that are chiral despite not having stereocenters. Conversely, not all molecules that contain stereocenters are chiral, since some of them contain planes of symmetry (we call these meso compounds). Therefore, it is best to become familiar with identifying symmetry in molecules, as this is the most conclusive way of determining whether or not they are chiral.
Another, albeit weaker, argument you could use is that the molecule contains no stereocenters, that is, it does not contain any carbon atoms with four different groups attached. It is a weaker argument because chirality has fundamentally to do with symmetry and there are molecules that are chiral despite not having stereocenters. Conversely, not all molecules that contain stereocenters are chiral, since some of them contain planes of symmetry (we call these meso compounds). Therefore, it is best to become familiar with identifying symmetry in molecules, as this is the most conclusive way of determining whether or not they are chiral.