when i input Item to inventory item, i got some of error like "A Child matrix item's parent must be a matrix item", "Uniqueness error - there is already an item with that name or name/parent combination."

That sounds like it is pretty straight forward

Uniqueness error - there is already an item with that name or name/parent combination

That's what it looks like at first glance.

when i check all item was active

He'e saying there is already a different item in the database with that name combination. You can use global search to find inactives by adding a

+

to the end of what you are looking for.