The site url rename is still under preview so it might not work for your tenant. Check in the New SPO Admin Center, when selecting a site and in the information panel, if next to the site url you have a Edit option. If so, you can try there.
Another workaround, but not really recommended, is also using SP Designer (again, not recommended).
Other than that, the ETA for full release on this is by end of this year, if all goes well.