If you're editing the back-half of a link and see the error This custom link is taken, you'll need to enter a different back-half. This is because someone has already used that back-half with that domain, whether it's s.id or your own custom domain.
Since s.id links are case sensitive, try changing the capitalization. You can also add other characters, like numbers or a hyphen.
For example, if someone has used s.id/sample, and you wanted to use the sample back-half, you could try s.id/example1 or s.id/example2022