GetItemByID error

Im making an inventory and was wondering why this error appeared

ItemDataBase' does not contain a definition for GetItemByID' and no extension method GetItemByID' of type `ItemDataBase' could be found. Are you missing an assembly reference?

This is my script that the error is telling me to fix:

Item itemAdd = itemdatabase.GetItemByID(id);

Im pretty young and learning to make games is hard for me but its fun so any help for me would be appreciated and yes i did watch a video to get the script and i don't see anything wrong with it