It is possible to implement an Ecommerce Bridge API without Sales Pro; the Ecommerce Bridge API technically only requires the integration-sync scope. The CRUD methods of the Products API can only be accessed by a portal with Sales Pro, but an integration with the integration-sync scope can create products by sending sync messages.
I'm having the same problem. I'm using Line Items API , which I understand is in preview. Our company has two HubSpot accounts. For one account I'm able to read line item with HAPI key without any problems. But for our second account I'm using OAuth authentication and I'm getting "Must have scope DEAL_LINE_ITEM_READ" error. Do you think that something must be set in our account?