Chances are the path to the jar file is incorrect.
Here is an example that works:
https://lux.vu/blog/?p=4
The text of the post contains:
<applet width="400" height="400" archive="/applets/interleaves.jar" code="interleaves" />
and the jar file is in an ‘applets’ directory at the root of my website. You could use a relative path also.
