@juniorzgt Check date/time settings on your computer.
If date/time settings are not preserved after a shutdown, then replace BIOS battery.
End if this error is caused by an expired SSL certificate. You need to update the SSL certificate to a newer one. Watch the detailed tutorial here: How to fix "your connection is not private" error on chrome
Fix SSL Certificate errors:
NET::ERR_CERT_DATE_INVALID
NET::ERR_CERT_AUTHORITY_INVALID
ERR_CERT_COMMON_NAME_INVALID
ERR_TOO_MANY_REDIRECTS
ERR_SSL_WEAK_EPHEMERAL_DH_KEY
ERR_SSL_VERSION_OR_CIPHER_MISMATCH
ERR_BAD_SSL_CLIENT_AUTH_CERT]
Before installing with the offline installer, rename the "Opera GX Stable" folder to "Opera GX Stable old" just to see if that sorts things out. If not, totally uninstall Bitdefender (don't just disable it) as a test and then try to install Opera to test if Bitdefender is the cause. You can install Bitdefender again then and if it was the problem, see if you can add exceptions for Opera, its profile and cache folders, and Opera's install folder etc.
Are you able to fully download the offline installer with another browser? If so, once it's downloaded, double-left-click it to launch it, click "options", adjust the options as you like, and install.
If some, please be specific about what fails and when, and what version of Windows.
@uknown11111 That's weird. Are you sure you downloaded the offline installer? The offline installing doesn't need to download anything at all. You can be totally disconnected from the net and install Opera with it.
@razin999 By default, Opera will start with the opened pages from last session. If you don't want it to happen, you can change the behavior in settings page.