Apologies for the inconvenience, there is an issue with 'Download Center for FPGAs' portal, we are working on it & we`ll get back to you soon.
Quartus Std/Pro/Lite 18.0 & Quartus II 13.0 SP1 Std Edition for windows platform are available in below old thread,
This issue fixed by our engineer internally. Please proceed with clearing your browser cache cookies & cache and then reopen your browser for login.
You should be able to download Quartus package after that.