Need help setting up raid on ncp

I am planning to create a home server on an odroid-hc4 using Nextcloudpi.
So far I managed to install and run the basic setup.
In order to have some data safety, I would like to use RAID1 using two HDDs. Unfortunately, mdadm cannot be installed. If I run sudo apt-get install mdadm it reports follwing:

Package mdadm is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'mdadm' has no installation candidate

How do I properly set up RAID1 on a nextcloudpi instance? Or install mdadm?

What image are you using for the OS?

I used the image from github.
After activation and the basic setup I run an update to nextcloudpi 1.54.2. I am not entirely sure about the patch (last digit) right now, but it is definitely 1.54.x.

Never mind.
Coming from an arch-based distro, I did not run apt-get update (updates package list) before attempting to install mdadm
After updating the package list it worked now.