What am I doing wrong here?

This feature is not available yet nor in your version nor in current openzfs version.

https://github.com/openzfs/zfs/blob/master/man/man8/zpool-attach.8#L48

You can't extend the existing raidz1 vdev by adding another disk but you can add another vdev to the pool to increase the pool's capacity. You'll need more than one additional disk if you want to retain redundancy. For example, you could use two disks to add a mirror vdev or 3+ disks for another raidz1 vdev.

TLDR: You cannot add devices to a RAIDZ… yet (edited bellow) !

This is an old limitation as mentioned by Dan Naumov in 2009

To reiterate, you cant just add a single disk drive to a raidz1 or raidz2 pool. This is a known limitation (you can check with SUN ZFS docs). If you have an existing raidz and you MUST increase that particular pool's storage capabilities, you have 3 options:

1. Add a raidz of the same configuration to the pool (think 3 disk raidz + 3 disk raidz or 5 + 5, for example)
2. Replace each (and every) disk in your raidz pool one by one, letting it resilver after inserting each upgraded disk
3. Backup your data, destroy your pool and create a new raidz pool with a bigger amount of disks.

But current docs still mention it:

zpool attach [-f] [-o property=value] pool device new_device

Attaches new_device to an existing zpool device. The existing device cannot be part of a raidz configuration. If device is not currently part of a mirrored configuration, device automatically transforms into a two-way mirror of device and new_device. If device is part of a two-way mirror, attaching new_device creates a three-way mirror, and so on. In either case, new_device begins to resilver immediately.

November 2023 edit : Hey Marty, we received news from the future!

RAIDZ extension is announced to be released in ZFS 2.3. It was a long run issue (~6 years), details in OpenZFS Developer Summit 2023 :

• the talk
• the slides