Hi,
sorry for the poor english, I had the same problem with my HP Microserver Gen 8 an PVE.
The problem is caused by the "issue" that you can't boot from the ODD Port (SSD) with the installed PVE.
Solution:
You need a Chainloader / GRUB Chainloader on a USB Stick to select the SSD as the boot-device.
Sorry, I am wrong.... ;-) You're at the installation-step, the bootloader is necessary for the boot-process after installation on the SSD.