The UCCB was replaced by the Canada Child Benefit (CCB) a couple of years ago. The CCB is non-taxable and doesn't need to be listed anywhere on your return.
There are taxpayers who have received back-pay from UCCB after the benefit ended. For this reason, CRA has elected to keep the line for reporting UCCB payments. However, if you didn't receive any back-pay for UCCB in 2018, only received the monthly CCB as usual, there's nothing for you to include on your tax return.