Open a terminal of your PVE host (be it via SSH or via the pve webui) and check which Debian release you are using. PVE 8 is based off of Debian 12 codenamed "Bookworm".

root@ethos:~# cat /etc/apt/sources.list
deb http://ftp.si.debian.org/debian bookworm main contrib

deb http://ftp.si.debian.org/debian bookworm-updates main contrib

# security updates
deb http://security.debian.org bookworm-security main contrib

deb http://download.proxmox.com/debian/pve bookworm pve-no-subscription

Here we add the HP signing key. Other tutorials provide two more keys (three in total), but those are - to my understanding - not required for this (ssacli) repository, so we'll only be adding one.

curl -fsSL https://downloads.linux.hpe.com/SDR/hpePublicKey2048_key1.pub | gpg --dearmor > /etc/apt/keyrings/hpePublicKey2048_key1.gpg
chmod 0644 /etc/apt/keyrings/hpePublicKey2048_key1.gpg

Add the repository to your APT sources (make sure you use correct OS release codename - in this case, at the time of writing, it is "bookworm"), update and install ssacli package

cat <<EOT > /etc/apt/sources.list.d/hp-mcp.list
deb [signed-by=/etc/apt/keyrings/hpePublicKey2048_key1.gpg] http://downloads.linux.hpe.com/SDR/repo/mcp bookworm/current non-free
EOT

apt update && apt install -y ssacli

You will most likely get the following warning after running apt update. You can safely ignore it as HPE (at least as of 31th of Oct 2024) have not migrated to the new format yet

N: Repository 'Debian bookworm' changed its 'non-free component' value from 'non-free' to 'non-free non-free-firmware'
N: More information about this can be found online in the Release notes at: https://www.debian.org/releases/bookworm/amd64/release-notes/ch-information.html#non-free-split

After successful installation you will be able to use the ssacli utility and (for example) list all drives in your system

root@ethos:~# ssacli ctrl all show

Smart Array P840 in Slot 3                (sn: PDNNF0ARH19A7M)

root@ethos:~# ssacli ctrl slot=3 physicaldrive all show

Smart Array P840 in Slot 3 (HBA Mode)

   HBA Drives

      physicaldrive 1I:1:1 (port 1I:box 1:bay 1, SATA HDD, 10 TB, OK)
      physicaldrive 1I:1:2 (port 1I:box 1:bay 2, SATA HDD, 10 TB, OK)
      physicaldrive 1I:1:3 (port 1I:box 1:bay 3, SATA HDD, 10 TB, OK)
      physicaldrive 1I:1:4 (port 1I:box 1:bay 4, SATA HDD, 10 TB, OK)
      physicaldrive 1I:1:5 (port 1I:box 1:bay 5, SATA HDD, 10 TB, OK)
      physicaldrive 1I:1:6 (port 1I:box 1:bay 6, SATA HDD, 10 TB, OK)
      physicaldrive 1I:1:7 (port 1I:box 1:bay 7, SATA HDD, 10 TB, OK)
      physicaldrive 1I:1:8 (port 1I:box 1:bay 8, SAS HDD, 2 TB, OK)
      physicaldrive 2I:2:1 (port 2I:box 2:bay 1, SAS HDD, 2 TB, OK)
      physicaldrive 2I:2:2 (port 2I:box 2:bay 2, SATA HDD, 2 TB, OK)
      physicaldrive 2I:2:3 (port 2I:box 2:bay 3, SAS HDD, 8 TB, OK)
      physicaldrive 2I:2:4 (port 2I:box 2:bay 4, SAS HDD, 8 TB, OK)

root@ethos:~#