Explain why BeH₂ molecule has dipole movement zero although BeH bonds are polar?

This is because in BeH2 molecule the effective dipole moments are cancelled out because they act on opposite directions. But in BeH the, H has to accept an electron from Be as it is more electropositive than H. Hence H developed a negative charge and there is a difference of charge. Hence it becomes polar.

The whole thing is based on the structure of the molecule which is linear take the case of ccl4 CCL bonds are polar but these can help very little on making the whole molecule with some effective dipole moment in this case is the structureis symmetrical. that is the main point