iWeb was a web creation app that was discontinued and dropped by Apple 9 years ago. It was a 32 bit app that worked with systems until High Sierra and Mojave when it started to become more3 and more incompatible.
With the release of Catalina and its incompatibility with 32 bit apps iWeb became one of many apps that left behind.
Today with Catalina and Big Sur there are many potential iWeb replacements, one of them being EverWeb which has a GUI similar to iWeb and will let you start with a blank page like iWeb could. It has a demo mode which will let you create your weblsite but won't be able to publish or save it outside of the app until the app is purchased.
The bottom line: iWeb can't be installed on Catalina or later and is a dead app.
