Why safari can’t establish secure network with zoom.us

What version of macOS is Safari running on >

Older version of macOS with older versions of Safari will have this type of issue.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence

Tips Big Sir Upgrade Released Nov 2020

Tips Monterey Upgrade  Released Oct 25, 2021

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal  operation of the OS and may even Corrupt the OS requiring a Reinstallation

When the iMac was purchased is not necessary the same Age as the computer.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

If the computer is running High Sierra macOS 10.13 - as per the Qualification set forth by Apple - it is probably running the most up-to-date version of macOS this computer Qualifies to run.

To verify information will require some work on the part of the User ( you ) to

1 - establish the Year of this computer. That method has been provided

2 - In the supplied link above and again here How to get old versions of macOS - verify the version of macOS this Computer Qualifies to run.