Some game data of a datapack or mod did not load

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

What it means

Datapacks and mods describe much of the game as data files: tags (groups of blocks or items), recipes, advancements and loot tables, each named <namespace>:<path>. When the server starts or /reload runs, Minecraft reads them all. A file it cannot parse, or one that names something that does not exist, is left out with an ERROR or WARN line, and the server keeps running.

What is lost is only that piece of data: a recipe that cannot be crafted, a tag that does not group what it should, a loot table that drops nothing or less than intended. A pack with many broken entries can print many of these lines; the diagnosis counts them per namespace.

Why it happens

The datapack or mod is made for another Minecraft version. Item ids, folder names (recipes/ became recipe/ in 1.21) and the JSON format change between versions.
The data names a mod that is not installed. A pack or mod adds recipes or tags for another mod’s items, and that mod is missing or renamed them.
A mistake in a hand-written file: invalid JSON, a misspelled id, a wrong criterion trigger.
Two packs disagree, for example one removes an item another’s recipe uses.

How to fix

Read namespace: it is the datapack or mod the data belongs to. For a tag, it is the namespace of the entry the tag is missing, which points at the mod or pack that should provide it.
Use a version of the datapack or mod made for the server’s Minecraft version.
If the data names another mod’s items, add that mod, or leave the error if nothing you need is missing in the game.
For your own datapack, open the file the line names (in world/datapacks/<pack>/data/<namespace>/), fix the JSON or the id, and run /reload.
If nothing is missing in the game, the warning can be left as it is: the rest of the server runs normally.

In Perland

The issue appears in the server’s Diagnosis tab with the namespace and the number of entries that failed. A datapack’s files can be fixed in the world’s datapacks folder in the Files tab; when the namespace is a mod of the server’s modpack, Mod Studio is where you update the mod or add the one it expects.

References

Details the diagnosis reports

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

Parameter	Meaning
`namespace`	The namespace of the data that failed, a datapack's or a mod's id (for a tag, the namespace of the first entry it is missing).

Log lines this page covers

Couldn't load tag <tag> as it is missing following references:
Parsing error loading recipe <id>
Parsing error loading custom advancement <id>
Couldn't parse element <type>:<id>
Couldn't parse data file <id>
Found loot table element validation problem in {loot_tables:<id>}