Why can't I Transfer ERC20 Tokens through my ERC721 contract as payment?

Question

Here's the function:

function transferERC20() external {
        _erc20.transferFrom(msg.sender, erc20Recipient, 6*10**18);
    }

After I imported the IERC20 Interface to interact with ERC20 tokens inside my contract in order to pay for certain functions using the ERC20 instead of ETH Value, I'm always left with a gas estimation error instead of a successful transfer. the balance of function works fine.

function getErc20Balance() external view returns(uint){
        uint balance = _erc20.balanceOf(msg.sender) / 1000000000000000000;
        return balance;
    } 

Answer 1

You can set up a contract to accept ERC-20 as payment for ERC-721 tokens.

Here is a demonstration of that technique

https://github.com/fulldecent/spend-ERC20-create-ERC721