Steps to reproduce:

1. Install zabbix_agentd 5.0.17
2. Run "zabbix_agentd -t web.certificate.get"

Result:
**

web.certificate.get                           [m|ZBX_NOTSUPPORTED] [Unsupported item key.]

Expected:

Works as expected 

This problem has been solved with the introduction of the new WebCertificate plugin available as a part of  Zabbix agent 2 5.4.4 release (also back-ported to Zabbix 5.0.15): 

https://mailchi.mp/2fb18ca718c6/zabbix-news-all-the-latest-updates-together-4858662?e=c5cd256baa

[tiago.cruz@vinhedo2 ~]$ zabbix_agentd -t agent.version
agent.version                                 [s|5.0.17]
[tiago.cruz@vinhedo2 ~]$ zabbix_agentd -t web.certificate.get
web.certificate.get                           [m|ZBX_NOTSUPPORTED] [Unsupported item key.]