Transaction reverted: function returned an unexpected amount of data while testing with ether.js (uniswapv2router.swapExactTokensForTokens function)

im testing UNISWAP_V2_ROUTER.swapExactTokensForTokens using ether.js and this line: await swapInstances.connect(accounts[0]).swap(tokenIn, tokenOut, amountIn, amountOutMin, to); cause this error : Transaction reverted: function returned an unexpected amount of data.

why?

unit test :

 it("should be able to swap tokens", async function () {
            accounts = await ethers.getSigners()
            to = await accounts[1].getAddress();
            const Swap = await ethers.getContractFactory("Swap", accounts[0]);
            const swapInstances = await Swap.deploy();
            const LocandaToken = await ethers.getContractFactory("LocandaToken", accounts[0]); //ERC20
            const locandaToken = await LocandaToken.deploy();
            const RubiconPoolToken = await ethers.getContractFactory("RubiconPoolToken", accounts[1]); //ERC20
            const rubiconPoolToken = await RubiconPoolToken.deploy();
            tokenIn = locandaToken.address;
            tokenOut = rubiconPoolToken.address;

            await locandaToken.connect(accounts[0]).transfer(swapInstances.address, amountIn);
            await rubiconPoolToken.connect(accounts[1]).transfer(swapInstances.address, amountIn);

            
            const ethBalance = await ethers.provider.getBalance(accounts[0].address);
            console.log("eth balance" + ethBalance);

            await locandaToken.connect(accounts[0]).approve(swapInstances.address, amountIn)
            const test = await swapInstances.connect(accounts[0]).swap(tokenIn, tokenOut, amountIn, amountOutMin, to);
        })

swap function :

function swap(
        address _tokenIn,
        address _tokenOut,
        uint256 _amountIn,
        uint256 _amountOutMin,
        address _to // address where sending the tokenout
    ) external {
        IERC20(_tokenIn).transferFrom(msg.sender, address(this), _amountIn); // transfer from user wallet to this contract
        IERC20(_tokenIn).approve(UNISWAP_V2_ROUTER, _amountIn); // aprove the router to spend _tokenin
        address[] memory path; //represents the path/flow of the swap

        if (_tokenIn == WETH || _tokenOut == WETH) {
            path = new address[](2);
            path[0] = _tokenIn;
            path[1] = _tokenOut;
        } else {
            path = new address[](3);
            path[0] = _tokenIn;
            path[1] = WETH;
            path[2] = _tokenOut;
        }

        IUniswapV2Router(UNISWAP_V2_ROUTER).swapExactTokensForTokens(
            _amountIn,
            _amountOutMin,
            path,
            _to,
            block.timestamp
        );

     
    }

You are deploying two new tokens. Hence, there are no uniswap pools available for those tokens yet.

I think you need to create a corresponding pair calling the uniswapV2Factory and then add liquidity: https://docs.uniswap.org/protocol/V2/reference/smart-contracts/factory#createpair

Second, where are you setting your amountOutMin?

third, Why are you transferring tokens to your swap contract?

await locandaToken.connect(accounts[0]).transfer(swapInstances.address, amountIn);

await rubiconPoolToken.connect(accounts[1]).transfer(swapInstances.address, amountIn);