Paste these lines into your server.properties (uses the recommended 1.21.4 version):
resource-pack=https://nexusprismpack.netlify.app/nexusprism-resourcepack-1.21.6.zip
resource-pack-sha1=4228bf5bc96bead3c30961c433cad9a645bbd214
resource-pack-prompt={"text":"NexusPrism requires this resource pack!","color":"aqua"}
resource-pack-required=true
Set CMD offset so fish items use the 3000–3999 range:
item-settings: custom-model-data-offset: 3000
{
"pack": {
"pack_format": 57,
"supported_formats": {
"min_inclusive": 15,
"max_inclusive": 99
},
"description": "NexusPrism Resource Pack"
}
}