I am trying install the plugin on an oracle-linux instance
I added the lifesteal-smp-plugin-1.6.4.0.jar file in /plugins directory
I ran the command java -jar lifesteal-smp-plugin-1.6.4.0.jar
I go the result:

no main manifest attribute, in lifesteal-smp-plugin-1.6.4.0.jar

Did I miss a step? or some dependency?

The install file was downloaded from Spigot

This is not an installer, you need to put the jar file into the plugins folder on your server and launch the server.