As it looks like, the plugin is not installed on your agent. This may occur due to many reasons. Please try to reinstall the plugin and check the agent's logs.
For example, one possible reason is the lack of memory in the agent. If you see errors such as:
INFO | jvm 1 | 2020/11/10 11:12:36 | 2020-11-10 11:12:36,120 ERROR [remoteEventRebroadcasterMessageListenerContainer-1] [ScanningPluginLoader] Unable to deploy plugin 'org.jfrog.bamboo.bamboo-artifactory-plugin' from 'Unit: /Users/yahavi/bamboo-agent-home/plugins/user-installed/plugin_8527020839401694412_bamboo-artifactory-plugin-3.1.0.jar (1604999532000)'.
INFO | jvm 1 | 2020/11/10 11:12:36 | 2020-11-10 11:12:36,121 ERROR [remoteEventRebroadcasterMessageListenerContainer-1] [ScanningPluginLoader] Because of the following exception:
INFO | jvm 1 | 2020/11/10 11:12:36 | java.lang.OutOfMemoryError: GC overhead limit exceeded
That means your agent has insufficient heap size. I'd recommend on the resolution in this guide: https://confluence.atlassian.com/bamkb/outofmemory-errors-in-bamboo-server-or-in-remote-agent-307038120.html
Edit:
For Elastic agents, you can use this guide:
https://confluence.atlassian.com/bamkb/outofmemory-errors-in-bamboo-elastic-agent-826899144.html