Degraded state means that either there is a failed/missing drive in the RAID or there is a mirror running that is not complete if there are no failed drives.
When a drive is failed, a hot spare must be used or a cold shelf spare must be swapped in to allow the RAID to rebuild and be complete.
1. If you do not have either of these options, you will need to
a.) Purchase a matching model
b.) RMA if the drive is in warranty
2. You can continue to work on a degraded RAID however we advise to only access data and not push a server to limits. Writing new data puts a degraded RAID at risk as new data is written across drives that are not complete. If another drive fails while degraded, there is potential data loss and data recovery services must be used to attempt to reconstruct the data.
Please see this document regarding data recovery services here.