hi omg! this happened to me too at first! it’s smth to do with tumblr and their restrictions with javascript. to fix it you’ll have to contact tumblr support for an exception to use javascript for your custom pages. that’s basically it. for tmi on what happens next read further:

when you do they’ll give you instructions on what to include in your next reply:
1. the url where you’re seeing the error message
2. the code you want to use (paste the code into pastebin and share the pastebin link)
3. the url for the page you’re trying to create (eg yourusername.tumblr.com/ursorter)
4. why you need the code

i know it’s so annoying and such a hassle but at least you only ever have to go through it once! after that you can make as many sorters freely! also they were pretty quick to respond! ( i think it was a day after)

if you have any more questions feel free to ask, and if you want like a screenshot of what my reply looked like feel free to ask for that too! (for the reason, i just said smth like i wanted to be able to make a ranker for me and others in fandom)

lmk how it goes!!