Can't Add A Category To Feature Groups

Hi all,
I've tried to add a new product category to feature groups, but when a try to save, CS-Cart shows me en warning: Couldn't assign feature to categories or a feature group. Some variations use this feature and aren't in the categories you tried to assign. Please delete these products, or disband their variation groups, or assign them to the same categories as feature or feature group, and try again.
I really don't get this warning. Someone could me explain in simple words, what's is going on with this?

Try not to add new category, but remove one of existent ones and add it back. Do you receive the same message?

Try not to add new category, but remove one of existent ones and add it back. Do you receive the same message?

On removing I receive the same message.