Uncaught SyntaxError: await is only valid in async function (its in an async function though....)

So the main issue here is, you're using await within the success function from $.ajax function (This is not possible, without making this function async as well, but now because your code is mixing callbacks/async, you're not able to resolve the original Promise of SolveRecaptchaV2. You're mixing async with old callbacks.) Your code is essentially doing:

SolveRecaptchaV2 => Ajax Request => Ajax Request Done, Firing callback
                         V                         V
                     Returning         Returning (But nothing cares about this return)

If you're using async functions, try to utilize async as much as possible. $.ajax will return a promise, so we can await that, and keep our function completely async:

function sleep(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

async function SolveRecaptchaV2(APIKey, googleKey, pageUrl, proxy, proxyType){
    var requestUrl = "https://2captcha.com/in.php?key=" + APIKey + "&method=userrecaptcha&googlekey=" + googleKey + "&pageurl=" + pageUrl + "&proxy=" + proxy + "&proxytype=";

    switch (proxyType) {
        case 'HTTP':
        requestUrl = requestUrl + "HTTP";
        break;

        case 'HTTPS':
        requestUrl = requestUrl + "HTTPS";
        break;

        case 'SOCKS4':
        requestUrl = requestUrl + "SOCKS4";
        break;

        case 'SOCKS5':
        requestUrl = requestUrl + "SOCKS5";
        break;
    }   
    try {
        await result = await $.ajax({url: requestUrl});

        if(result.length < 3) {
            return false;
        } else {
            if(result.substring(0, 3) == "OK|") {
                var captchaID = result.substring(3);

                for(var i=0; i<24; i++){
                    var ansUrl = "https://2captcha.com/res.php?key=" + APIKey + "&action=get&id=" + captchaID;  

                    var ansResult = await $.ajax({url: ansUrl});
                    console.log(ansResult);
                    if(ansResult.length < 3) {
                        return ansResult;
                    } else {
                        if(ansresult.substring(0, 3) == "OK|") {
                            return ansResult;
                        } else if (ansResult != "CAPCHA_NOT_READY") {
                            return ansResult;
                        }
                    }
                    await sleep(1000);
                }
            } else {
                // ansResult is not defined here, not sure what you want to return here (May want to return false, or an empty string):
                return ansResult;   
            }
        }
    } catch (err) {
        //On ajax failure, return empy string. (May want to return false here, to fall inline with your "if (result.length < 3)" statement above.)
        return "";
    }
}

Now our chain looks like this:

SolveRecaptchaV2 => Ajax Request
                     => Ajax Request Done
                       => Get Answer URL
                         => Sleep
                           => Get Answer URL
                             => Sleep (...loop)
                               => Resolve promise of SolveRecaptchaV2.