cannot generate bitcoin first block

Question

I have download bitcoin source and change genesis in regtest. and i try to generate 1 block by command ./bitcoin-cli -generate 1 it does not show any mining block. i try over 10 times but it is still the same result why?

here the result:
{
  "address": "tc1qltnl9t23yshlexxxxxxxxxxxx",
  "blocks": [
  ]
}

please kindly advise what happen what bitcoin source code ?