ERR_HS_ASSOCIATION on HubSpot: HubSpot API v3 Association Calls Return 400 Bad Request. Root cause: HubSpot API v3 association request returned 400 Bad Request due to invalid object types, malformed association type, or unsupported association. Common causes: object type not recognized, association type ID invalid, attempting to associate same object to itself, missing required fields in request, API version mismatch. Step 1: Retrieve Valid Association Type IDs. Call GET /crm/v4/associations/{fromObjectType}/{toObjectType}/labels to get the list of valid association types for your object pair. Note the associationTypeId values for HUBSPOT_DEFINED associations. Step 2: Update Your API Payload. Change your association creation payload to include the associations array with associationCategory: "HUBSPOT_DEFINED" and the correct associationTypeId. For example, Contact-to-Company uses typeId 1, Deal-to-Contact uses typeId 3. Step 3: Use Batch Association Endpoints. For bulk association operations, use the POST /crm/v4/associations/{fromObjectType}/{toObjectType}/batch/create endpoint instead of individual calls. This reduces API calls and avoids rate limiting.