Many web sites will not work with old browsers or MacOS versions. Changes in websites' security certificates in Sept 2021 means that many Macs running macOS 10.11 as you appear to have cannot connect. If your iMac will handle a higher macOS version, that will help. Most sites still work with macOS 10.13.
I have an older iMac...
"Older" is pretty vague when talking iMacs. They go back as far as 1998! We need to tighten that identification:
The best place to start is to do "About this Mac...' for your Apple menu and tell us your iMac's SUB-MODEL. That alone determines if you have any OS upgrade options remaining. As ALL of the sub-model model name is important to macOS compatibility with your iMac, please include and modifiers such as Early, Mid, or Late when you report back. This is the "about this Mac" results for one of our older iMacs running OS 10.11:

- Your iMac must be a Late 2009 or newer to run 10.13 High Sierra.
- Your iMac must be a 2012 or newer to run anything higher.