The server type does not offer the version the server asks for

Your log will have other names, versions and times. Search a line of it from all failures.

What it means

The server’s Docker image installs the server software at startup with its helper tool. Before it downloads anything, the helper checks that the software offers the version you asked for. It did not, so the helper refused, nothing was installed, and the server could not start.

Each server type words the refusal its own way: Paper names the version and build, vanilla and Quilt say the Minecraft version is not valid, Forge says the version is not available from Forge, and NeoForge says it cannot find a suitable version. Spigot, Bukkit and Fabric do not print such a line: their download answers with an error instead, which is a failed download (source_download_failed).

Why it happens

A typing mistake in the version, such as 1.21.40 for 1.21.4.
The software has no build for that version yet. A brand new Minecraft release often takes days or weeks to reach Paper, Forge or NeoForge.
The software never supported that version. Not every loader covers every Minecraft release.
A Paper build or NeoForge version that does not exist was pinned.

How to fix

Check the version for typing mistakes.
Look up which versions the software offers: the Paper downloads page, the Forge or NeoForge file lists, or the Minecraft launcher for vanilla.
Pick a version that exists. If you pinned a Paper build or a NeoForge version, remove the pin or pick one that is listed.
Changing to an older Minecraft version than the world was last saved with can damage the world. Make a backup of the world first.

In Perland

The issue appears in the server’s Diagnosis tab with the version it asked for. Pick another version in Settings: the version list there shows the versions each server type offers.

References

Details the diagnosis reports

Perland's server diagnosis returns this failure as the code version_not_available with these parameters, each present only when the log names it.

Parameter	Meaning
`command`	The startup helper's command that refused the version, such as install-paper, install-forge, install-neoforge, install-vanilla or install-quilt.
`version`	The Minecraft version (or the proxy's version) that was asked for.
`build`	The Paper build that was asked for; present only when a specific build was requested.
`loaderVersion`	The NeoForge version that was asked for; present only when one was set (absent when the latest was asked for).

Log lines this page covers

Invalid parameter provided for 'install-paper' command: Requested version ... is not available
Requested version ..., build ... is not available
Minecraft version '...' is not valid
Minecraft version ... not available from Forge
Unable to find suitable version for Minecraft ... NeoForge ...
Version ... does not exist