Beginner question on Updating List Items using JSOM, REST and Sharepoint Online

I tested your script in my online environment, it works fine.

For error "No item exists...", confirm the ID exists.

The error "Input string was not in a correct format" caused by you got wrong ID parameter as there are other parameters, you could use below script to get parameter instead.

function getQueryStringParameter(paramToRetrieve) {
            var params = document.URL.split("?")[1].split("&");
            for (var i = 0; i < params.length; i = i + 1) {
                var singleParam = params[i].split("=");
                if (singleParam[0] == paramToRetrieve) return singleParam[1];
            }
        }

Update:

Try combine as below:

SP.SOD.executeFunc('sp.js', 'SP.ClientContext', function()
        {
    var itemId =getQueryStringParameter('ID');
    var ctx = new SP.ClientContext.get_current();
    var customList = ctx.get_web().get_lists().getByTitle('Tasks');
    var listItem = customList.getItemById(itemId);
    listItem.set_item('formStatus', 'Completed');
    listItem.update();
    ctx.executeQueryAsync(
        function(){ alert('Item updated'); }, 
        function(sender, args){ alert('Error: ' + args.get_message()); }
    );
    });
function getQueryStringParameter(paramToRetrieve) {
            var params = document.URL.split("?")[1].split("&");
            for (var i = 0; i < params.length; i = i + 1) {
                var singleParam = params[i].split("=");
                if (singleParam[0] == paramToRetrieve) return singleParam[1];
            }
        }

SP.List.getItemById() takes a number as an argument, but your query string parameter is a string. (Documentation here.)

Try

var itemId = parseInt(getQueryStringParameter('ID'));

or

var itemId = Number(getQueryStringParameter('ID'));