Spare disk can't be used to replace bad disk

[thunderlight1]

Hi!
I am having some issues with a disk (ada2) as shown in the Alerts notices, as shown bellow:

I have a pool of two vdevs in raid1 config as shown bellow:

As you can see, the spare is ada4, but when I try to replace the faulty disk I can only select ada5 as shown bellow:

Bellow are all my available disks:

Would appreciate if I can get any tips on how to replace the faulty disk with the spare disk.