Based on the Rest API documentation, an 'item' isn't a valid endpoint: https://system.netsuite.com/help/helpcenter/en_US/APIs/REST_API_Browser/record/v1/2024.2/index.html.

If you were to update an assemblyItem your call would be a patch to: https://[accountid].suitetalk.api.netsuite.com/services/rest/record/v1/assemblyItem/{id}