Ok, found what was happening but I cant seem to understand it, on the form, under

sublistfields

there is a SS that is filtering those items that are not set, I did a test, deactivated the search and it let me post the item, but what I dont get is why is not letting post an item, on the help it just says that this option just doesnt show the item, I would think that even when is not shown, the item can be posted.