change multiple fields with one button

It actually is saying you aren't getting an error (thats why it says No error in the middle). This means it is not getting to your redirect and doesnt know where to go. However, your syntax to change multiple fields is simpler than you are making it. You do not need to call the edit record api multiple times. Rather:

var text Edit =  
URLRoot() & "db/" & Dbid() & "?a=API_EditRecord&rid=" & $RID
& "&apptoken=myapptokenhere
& "&_fid_79=" & URLEncode("Cancelled")
& "&_fid_65=" & URLEncode("Closed");

A couple things to note.

1. Good practice not to post your app token as it is an extra security level and best to keep that confidential.
2. You can keep adding as many fields as you like to set.
3. The URL Encoding is never a bad idea and won't hurt anything, but really is designed to be sure to take care of special characters (so when in doubt, use it), but in your case if you wanted, you could simplify to:

var text Edit =  
URLRoot() & "db/" & Dbid() & "?a=API_EditRecord&rid=" & $RID
& "&apptoken=myapptokenhere
& "&_fid_79=Cancelled" 
& "&_fid_65=Closed";