It's been a while, but we had the same issue while evaluating a license for our Elastic installation. Same behavior as here; Security was just not available and we just got the "Application Not Found" error message.
Turns out this is caused by using the deprecated(!) setting telemetry.enabled: "false" in kibana.yml. This was changed to telemetry.optIn: false + telemetry.allowChangingOptInStatus: false and some validation errors caused Security (as well as other plugins) to be deactivated when telemetry was disabled in the old way. I am not going to go into further detail — there is a GitHub PR that you can reference if you want more information: Gracefully handle deprecated telemetry config by eokoneyo · Pull Request #266029 · elastic/kibana
So in short: Should be fixed soon. Best fix is to update the telemetry option in kibana.yml.