I’m not sure where you are getting this line from, but it looks to be an issue with the recent migration to the polkadot-sdk monorepo. There is no “polkadot-v1.0.0” branch there ( Tags · paritytech/polkadot-sdk · GitHub ).
Maybe try “polkadot-v1.1.0” instead? The nicks pallet seems to be there - https://github.com/paritytech/polkadot-sdk/tree/polkadot-v1.1.0/substrate/frame
Or read it from the Substrate repo instead? GitHub - paritytech/substrate: Substrate: The platform for blockchain innovators instead of polkadot-sdk.git
Please report back if either of these work and ideally file a PR or issue where you saw this line originally.