The error you encountered, "On-Open order cannot have a post-only attribute," stems from the misuse of the post-only attribute. In QuantConnect, the post_only attribute is designed specifically for Limit orders executed on certain exchanges, ensuring no part of the order takes liquidity (thus potentially rejecting any liquidity-taking portion). This attribute does not apply to Market On Open (MOO) orders.

Here are the steps to resolve the issue:

1. Remove the post-only Setting: Since the post_only attribute is only meant for Limit orders and your usage involves a Market On Open order, you should remove any setting related to post_only. Your order properties should omit any configuration for post_only. Here's how you can update your code:

   # Initialization: No need to set post_only for MOO orders
   def initialize(self):
       self.default_order_properties = OrderProperties()
   # Order: Use market_on_open without order_properties involving post_only
   def place_order(self):
       self.market_on_open_order(self.symbol, self.quantity)

2. Review Your Order Requirements: Ensure that your usage of Market On Open orders adheres to both QuantConnect's and IBKR's requirements. Market On Open orders need to be submitted to participate in the official opening auction. They typically don't require any additional properties like post_only.

3. Debug and Verify: If the problem persists, you should use logging or debugging tools to verify the parameters being sent with your order to ensure they are as expected.

4. Documentation Review: Consider reviewing IBKR's specific details on Market On Open orders to verify compliance without unnecessary attributes.

Make these adjustments, and your MOO orders should work without encountering the "post-only" attribute error.

Sources: Market On Open Orders - QuantConnect.com, Order Errors - QuantConnect.com