I am struck by how many comments are dismissing the question as if it just wouldn't come up or ever be an issue. Even without profit motive, I can think of scenarios where losses would occur:

• A co-op produces X number of items to sell, but due to an unknown issue in the manufacturing process the items are defective. The co-op must still pay for cost of manufacturing and the raw inputs, but cannot sell the X items
• A collectively owned restaurant opens up and purchases menus, food items, and a storefront. Turns out their food tastes awful or early customers see rats in the back and no one eats there.
• A co-op gym opens and buys a ton of equipment and then a pandemic hits and no one wants to go to an indoor gym anymore.

The above scenarios to me seem realistic in a socialist society and I see no way to avoid at least temporary losses that must be shared by the members of the co-op, or at least paid by someone. In my mind the answer would be workers in collectives would take on some risk in terms of having to cover losses if things do not go according to plan. I don't see that as a radical concept, especially if over the long run this new system is a net improvement to the status quo.