Hi Pren51.
Welcome to the Apple Community.
Seems the error could be coming up for any one of various reasons.
You. might want to learn whether others can access the site ok, or whether no-one else can access that site either.
Some possible reasons:
May be Security issues: Website may be being protected .... unauthorised access not available.
Eg: the website barring entry at the time (eg: because of hackers, or website currently unavailable / being updated etc.)(You could try communicating with the Company / Business / Agency behind the website)
May be because your Mac & devices software is not updated to the standard now needed to access the website.
May be something to do with the settings, preferences, or other. data on your device
eg: ... necessary cookies missing for that site ... or cookies preventing access ....
or something in Safari's History .... or website Data ... could be blocking your attempts
May be a firewall issue. (either the website company''s or yours)
May be Network Settings : if so, resetting might help.
Sorry there seems no simple answer.
All the best :-)