Cannot import other contracts

[DerekJarvis]

I'm trying to import either a library or an existing contract to debug and I always get this error:

Member "..." not found or not visible after argument-dependent lookup

Here's a simple example using this library: https://github.com/rob-Hitchens/UnorderedKeySet/blob/master/contracts/HitchensUnorderedKeySet.sol

Test2.t.sol

// SPDX-License-Identifier: MIT
pragma solidity >= 0.4.21 < 0.9.0;
import "contracts/HitchensUnorderedKeySet.sol";

contract DbgEntry {
    event EvmPrint(string);

    HitchensUnorderedKeySetLib.Set private listLookup;

    constructor() {
        emit EvmPrint("DbgEntry.constructor");

        listLookup.insert(bytes32(0));

        uint256 abc = 123;
        uint256 def = abc + 5;

        emit EvmPrint("DbgEntry return");
    }
}

When trying to debug a contract I'm using the example provided of creating a new instance of it within the test contract. The error is the same.

What am I doing wrong?