What behaviour is observed?

Server stop immediately after start.

[18:23:46] [Server thread/INFO]: [AuthMe] Enabling AuthMe v5.6.0-bCUSTOM
[18:23:46] [Server thread/WARN]: [AuthMe] You are running an unsupported server version (CraftBukkit v. 4312-Bukkit-0fb24bb (MC: 1.21.1)). AuthMe requires Spigot 1.8.X or later!
[18:23:46] [Server thread/WARN]: [AuthMe] THE SERVER IS GOING TO SHUT DOWN AS DEFINED IN THE CONFIGURATION!

Expected behaviour

If server is Spigot 1.21.1 instead of CraftBukkit 1.21.1, the plugin works fine.

To Reproduce

1. Get BuildTools.jar from spigot website
2. Run java -jar BuildTools.jar --compile craftbukkit
3. Get craftbukkit-1.21.1.jar from compile result

Run CraftBukkit instead of Spigot is not a mistake. And plugin is downloaded from bukkit website https://dev.bukkit.org/projects/authme-reloaded/files/5484191 instead of spigot website https://www.spigotmc.org/resources/authmereloaded.6269

Plugin list

plugins folder contains only AuthMe-5.6.0.jar, the server stops immediately, so no /pl can be executed by admin player or from console.
AuthMe folder is not generated after server start and stops.

Server Implementation

Standalone server (no proxy)

Database Implementation

SQLite

AuthMe Version

5.6.0 I assume because no command can be executed

Error log (if applicable)

Provided in "What behaviour is observed?" section

Configuration

Blank server no config

Get authme-5.7.0-SNAPSHOT.jar from https://ci.codemc.io/job/AuthMe/job/AuthMeReloaded and the result is the same.

This is intended behavior. CraftBukkit only exists for legacy reasons, while Spigot gets the feature and API changes.
Therefore AuthMe is build againts the Spigot API, requiring the server to run at least Spigot.