Replying to Fantasy Football Roster Error

I have a fantasy football team on yahoo and I have been getting these error messages the past few weeks whenever trying to put a player that I have had on my team all season into an open roster spot. Due to the open spots I have been loosing points the last few weeks. The exact error messages are:

You did not give a position for a player. (Error #158)
That position has already been filled. (Error #159)
You have an internal roster error. (Error #290)