Can't add Main Category

I have a product without a main category (don’t ask me how it got that way). When I try to add a main category, I click on the little icon that pops up the category list, then i scroll through and select a sub-category of a category (ex: Clothing → Shirts). It shows up in the Main Category textbox. Then I click save. The main category does not save, it goes back to a blank textbox.

Does anyone know what may be causing this issue?

it may be the uncompleted DB transaction cause your product lost main category in the record.

I guess, the SQL update statement on CS-Cart product page can’t update product category record with unknown or empty category ID. That is why it go back to a blank category for this product.

If you know how to use the phpMyAdmin, you can just find your product ID and Main category ID and update it directly at cscart_products_categories table on your mysql DB. That will solve your problem more faster!!!