script params doesn't support values of type: START_ARRAY in ElasticSearch v7.3 Update script
Your params section must not be an array, but an object, use this script instead:
{ "script": { "source": "ctx._source.hasParts.add(params.haspart)", "lang": "painless", "params": { "haspart": [ { "documentLevel": 3, "levelProperties": { //some properties } }, { "documentLevel": 3, "levelProperties": { //some properties } }, { "documentLevel": 3, "levelProperties": { //some properties } }, { "documentLevel": 3, "levelProperties": { //some properties } }, { "documentLevel": 3, "levelProperties": { //some properties } } ] } }}