Hi @Michael Williams ,
Please try to add jquery.min.js and jquery.SPServices-2014.02.min.js into a document library, then reference the files.
And add try...catch into your code.
try {
adddlert("Welcome guest!");
}
catch(err) {
document.getElementById("demo").innerHTML = err.message;
}
For sharepoint 2016, I will recommend REST API to achieve your requirement
https://social.technet.microsoft.com/wiki/contents/articles/31995.sharepoint-2013-working-with-rest-api-using-jquery-ajax.aspx
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.