General Question about payable

[Frankrory113]

function fund() public payable{
        // msg.value.getConversionRate();
        // Want to be able to set a minimum fund amout in USD
        // 1. How do we send ETH  to this contract?
        require (msg.value.getConversionRate() >= MINIMUM_USD, "Didn't send enough"); 
        funders.push(msg.sender);
        addressToAmountFunded[msg.sender] = msg.value;
        // 1e18 == 1 * 10 ** 18 ==1000000000000000000
        // 18 decimals
        }

I got question on this ,with a error report

TypeError: "msg.value" and "callvalue()" can only be used in payable public functions. Make the function "payable" or use an internal function to avoid this error.