I’m unable to load an Applet I made using BlueJ, and host it, or use it from a home/local server IP as a Java web server. I tried searching for the error message, and I didn’t get any helpful results.

I’ve gotten Java servers and clients to work just fine before, but this is the first time for a web page.
Can anyone help me?
Let’s start at the beginning: Java Applets are kind of a dead technology, and have been for a while, so if you’re trying to learn Java you may just want to skip over them entirely.
Historically you’d indeed run them as part of a web page through a browser plugin, though none of those still work with recent browser versions (iirc Firefox has some LTS version that explicitly supports ancient stuff, so that may still work)
There used to be an applet viewer included in the JDK that was able to run them “standalone” but I assume you’d need an old JDK (JDK8 or older) for it to still be included.
It’s weird. I can open the file with Internet Explorer, and Mozilla Firefox, and load, and run the file just fine, but when it’s done with/over the web server, it just gives that error message for both browsers. I can’t figure out a way around it.
Even when I switch out of/stop using BlueJ, and run the web server through Command Prompt, it’s still the same issue/error message.