Can you please provide the twurl request / response when you received this error? I tested myself creating a line item with objective=FOLLOWERS and bid_type=TARGET without issue. Thanks!
$ twurl -X POST -H ads-api.twitter.com "/2/accounts/18ce5475gij/line_items?campaign_id=a9072&objective=FOLLOWERS&product_type=PROMOTED_ACCOUNT&placements=ALL_ON_TWITTER&bid_amount_local_micro=1000000&entity_status=PAUSED&bid_type=TARGET" | jq
{
"data": {
"bid_type": "TARGET",
"advertiser_user_id": 1877861928,
"name": "Untitled",
"placements": [
"ALL_ON_TWITTER"
],
"start_time": null,
"bid_amount_local_micro": 1000000,
"automatically_select_bid": false,
"advertiser_domain": null,
"target_cpa_local_micro": null,
"primary_web_event_tag": null,
"charge_by": "FOLLOW",
"product_type": "PROMOTED_ACCOUNT",
"end_time": null,
"bid_unit": "FOLLOW",
"total_budget_amount_local_micro": null,
"objective": "FOLLOWERS",
"id": "awypl",
"entity_status": "PAUSED",
"account_id": "18ce5475gij",
"optimization": "DEFAULT",
"categories": [],
"currency": "USD",
"created_at": "2018-01-23T00:41:34Z",
"tracking_tags": [],
"updated_at": "2018-01-23T00:41:34Z",
"include_sentiment": "POSITIVE_ONLY",
"campaign_id": "a9072",
"creative_source": "MANUAL",
"deleted": false
},
"request": {
"params": {
"bid_type": "TARGET",
"placements": [
"ALL_ON_TWITTER"
],
"bid_amount_local_micro": 1000000,
"product_type": "PROMOTED_ACCOUNT",
"objective": "FOLLOWERS",
"entity_status": "PAUSED",
"account_id": "18ce5475gij",
"campaign_id": "a9072"
}
}
}