EqualLogic drive faled. Array has fewer spare drives than expected.

Hi. 

We have PS6100 EqualLogic with 24 SSD drives in total (RAID10, 1Pool). One drive is as Spare. Now one disk failed and we see message "Array has fewer spare drives than expected". Does that mean we are not able to use that spare ("Add disk to RAID" is not active)?

Why we can add spare to RAID?

Is any way to replace failed drive by the spare, or it's the only option to buy new disk and replace failed disk?

Thanks