Hi,

The error message appears because Odoo’s Subscription + eCommerce integration prevents customers from adding incompatible subscription products to the same cart. For example, you cannot mix monthly and yearly subscriptions or multiple subscriptions from the same template. Even when the cart looks empty, Odoo may still detect a conflict.

This issue usually happens if there is still a draft sale order linked to the customer that includes a subscription product. Sometimes canceling is not enough, and the draft order must be deleted. Another common cause is that the cart session in the browser is not fully cleared, so the system thinks a subscription is still in the cart. In some cases, the subscription products themselves belong to the same subscription template, which Odoo treats as incompatible.

To resolve it, you should first check the draft sale orders/carts for that customer and delete any that contain subscription products. Next, clear the website cart session by logging out or clearing the browser cache. Finally, verify the subscription product setup to ensure that no conflicting templates are assigned. If the issue happens for one customer but not others, it confirms the problem is data-related.

Hope it helps

That error usually means a leftover subscription session—try clearing browser cache/cookies and recheck draft orders before adding again.

Possible Causes & Fixes:

1. Multi-World or Domain Mismatch

If you're accessing the eCommerce site via different domains (like www. vs non-www.), session cookies and cart data might not sync properly.

Fix:

Ensure a consistent domain; redirect one version to the other and verify that cookies are shared correctly across subdomains.

2. Product Configuration (Attributes & Optional Products)

Products with variants or configurators might trigger modals or blocking behavior instead of adding to the cart.

Fix:

If using optional or configurable attributes:

• Ensure the necessary module is installed (e.g., website_sale_product_configurator for optional products
  
• Make sure optional products are published and correctly setup.

3. Miscellaneous Add-to-Cart Behaviors

If you're seeing duplicate lines or odd application logic, it could be due to attribute settings or user configuration.

Fix:

Look into shop settings under Shop - Checkout Process to adjust how "Add to Cart" operates, such as merging lines or updating quantities.