Installing ElasticSearch plugins
The plugin command doesn't do anything special, it just tries to download the zip file from different locations and extracts it to a specific subfolder under plugins. Most of the plugins used to be available directly as GitHub downloads, but since GitHub has disabled its downloads feature, there is a discussion going on about what's the best way to work around this. The plugins coming from the elasticsearch team have been moved to download.elasticsearch.org and the plugin command has been lately updated to reflect this change. Have a look at the related issue and this blogpost about it.
Anyway, if you have problems using the plugin command you can just download the zip file and extract it to a specific subfolder under plugins. After that you need to restart your elasticsearch node and you'll see in the output a list of plugins that have been found and succesfully loaded during startup. Given your explanation I guess you didn't create a knapsack (or any other name you prefer) folder under plugins. Just create it and put the jar within that folder.