Why can't a user import keys to the keypool for a blank wallet?
When you create the wallet that you are importing your keys into, presumably with something like bitcoin-cli -rpcwallet='mywallet' importmulti, it needs to be created in watchonly mode, meaning with private keys disabled. To do that, create the wallet with bitcoin-cli createwallet 'mywallet' true, where true is the argument for disable_private_keys, creating a wallet for watch-only (public) keys.