computer wont allow YW to be " Ministered to"

Why cant I assign a RS sister to "Minister to" a young women? They have "ministering Brothers" assigned to them, yet when I try to attach a ministering sisters to them, the computer will not allow it.

Your bishop or stake RS leaders would be good people to ask that question. Until someone who knows the real answer replies, there is language in Handbook 2 (publicly viewable) that appears to shed some light on the question. In section 9.5.2, bullet point 1, second sentence, it says this: "The Relief Society presidency recommends ministering companionships and ministering assignments for Relief Society sisters." At least in earlier years, YW were not considered "Relief Society sisters."

Ministering Sisters minister to members of the Relief Society. I believe the EQ only assigns to families, although I found some verbage in the Handbook that casts some doubt on that.

But for reference:
"A Mia Maid or Laurel serving as a companion to an adult sister does not have ministering sisters assigned specifically to her. She is ministered to by those who minister to her family and is also cared for by her Young Women leaders." Handbook 2: 9.5.3