When requesting a sub, and their name is grayed out, you will be unable to choose them for the selected dates and/or requirements you have set. This is because the sub has an overlapping commitment and cannot accept your request.
In the example below, I have requested a sub with a credential or permit only. My only available sub would be Candice because she has a credential on file. If Candice was also greyed out, that would indicate that she is unavailable for the dates requested because she is already working another request on one or many of the days you are requesting.
You can choose another sub or submit the request to the sub pool in your area. Any available sub, that meets your requirements, can then accept your request.