Huh! If you go to the System Console, Plugin Management, do you see the Zoom plugin as green and running? What version is it? Maybe poke around in /opt/mattermost/plugins and look for the Zoom plugin and see if permissions look correct, etc?
A search turned up [SOLVED] Custom Slash Command - Command with a trigger failed which suggests: If anyone wants the solution: AllowedUntrustedInternalConnections in config.json needs to contain the hostname of the webhook. That seems odd in this case, but worth a try!