This might have something to do with it but I am not 100% sure:
Given that both Chrome and Opera are not open source, I don’t think there can be much guarantee with them working flawlessly.
Can you post pictures (e.g. screenshot) of the exact error you’re seeing from the Qt installer? I don’t think we can give much feedback but I can at least try to help in pinpointing why it’s failing. My gut is telling me it’s likely due to the release being EOL but it’s also too soon to say.
Also, can you let me know where you got the packages for Opera and Chrome from? I’m assuming you got them directly from their sites given they aren’t in the repos. Opera is available as a snap btw.
Alternatively, you can install Chromium (the project Google Chrome is based on) instead of Chrome.
You can see the following Lubuntu manual page for info on getting both Opera and Chromium via Discover (software catalog):
https://manual.lubuntu.me/stable/4/4.1/discover.html