Reboot into Safe Mode, and try again.
This disables various add-ons that can get in the way.
Common sources of problems are add-on anti-malware, add-on anti-virus, add-on firewalls, add-on cleaners, etc.
Or there’s some malware installed.
It’s also possible you’re on a network with filtering enabled, such as that of a school or business.
If so, follow the organization's policies, and contact the organization's IT folks for assistance here.
How old a version of macOS are you running? OS X 10.11 and earlier are getting dicy.
Failing the above, download and run EtreCheck, then open a new reply here and then press the button that looks like a printed page to get a text input box big enough to paste the hardware and software configuration report here.