MobileMe and iWeb were discontinued by Apple over 7 years ago. And for over a year we would get a warning like this whenever we launched a 32 bit app that ould not run on Catalina:
You have two options:
1 - revert back to your last system: Revert your Mac to a previous macOS version
2 - start migrating your website to another, supported, 64 bit app. One that many went to is EverWeb because of it's similar GUI and drag a drop capability. There's a demo version of it you can try before purchasing. You can start replacing pages one at a time with the new app, making the transition a bit easier.
Visit the App Store and search for website or website design, ete. There will be a number of possible candidates.