Source: https://docs.prylabs.network/docs/advanced/proof-of-stake-devnet
$ ./prysmctl testnet generate-genesis --num-validators=64 --output-ssz=genesis.ssz --chain-config-file=config.yml --override-eth1data=true
INFO[0000] Specified a chain config file: config.yml prefix=genesis
FATA[0000] Could not generate beacon chain genesis state error="could not set config params: version 0x04000000 for fork deneb in config interop conflicts with existing config named=mainnet: configset cannot add config with conflicting fork version schedule" prefix=genesis