Install plugins on a Paper or Purpur Minecraft server

Use this guide to install a Paper or Purpur plugin, check that it works and find the cause when it fails. Start with one plugin at a time so you can tell which change caused a problem.
Check the plugin before downloading
Read the developer's release page. Match the plugin to your Minecraft version, server software and Java runtime. A project may offer separate downloads for Paper, Velocity and Fabric. A Fabric, Forge or NeoForge mod will not work as a Paper plugin. Use the developer's official site or project listing; avoid cracked JARs and files sent by strangers.
Check the dependency list too. Some plugins need another plugin before they can start. Keep the previous working JAR and configuration outside the active plugins folder if you intend to roll back.
Install or update a plugin
Sign in at game.cloudexa-hosting.com, select the correct server and create a backup. Export any external database that the plugin uses.
Open Console, stop the server and wait for it to finish saving.
Open Files and the 
plugins folder. Confirm that your server runs Paper or Purpur if that folder is missing.
Choose Upload and select the plugin's 
.jar. If the download is a ZIP, inspect the author's instructions and upload the correct JAR from it.
Upload any required dependency. When updating, move the previous version outside 
plugins so the server loads one copy.
Start the server. Read the first startup messages for the plugin, then run 
plugins in Console without a leading slash.
Files shows the world folders, plugins and server.properties. Example data; use your own server files. Select the image to view it full size.
Configure it and test as a player
Many plugins create a folder in 
plugins after their first start. Stop the server before editing its configuration unless the author gives a supported alternative. Preserve YAML indentation and use spaces instead of tabs. Save your changes, restart, and test the feature with a normal player account as well as an administrator account.
A plugin can appear in the list while a database, permission or optional feature still fails. Check the action you installed it for. Use the author's documented reload command only where supported; Minecraft's general 
/reload command can leave plugins in an inconsistent state.
Find the first useful error
Open 
logs/latest.log through Files, or follow the log collection guide. Read from the start of the failed plugin's loading sequence. Later errors can follow from the first failure.
Missing dependency: install the named dependency and check its supported version.
Unsupported API or Java version: compare the plugin requirements with the server and Java versions in the startup log. Changing a world to an older Minecraft version can damage it; ask support before attempting that.
Invalid configuration: check the file and line named in the error. Compare your edits with the author's example, keeping a copy before changing it.
Database timeout or login failure: check the database endpoint, port, database name and allowed connections. Share error text through a private ticket, with passwords removed.
Duplicate plugin or version conflict: inspect the folder for old JARs and check the developer's compatibility notes.
Roll back a failed installation
Stop the server and move the new JAR outside 
plugins. Keep its data and logs. Start again and compare with the previous working state. A plugin update may also change its database or data files, so restoring an older JAR alone may not undo the change. Contact support with the plugin name, download link, version and first error before restoring a backup.
Reference: Paper's plugin installation and troubleshooting documentation.