First off, no bootable device generally has nothing to do with FreeNAS. That's a BIOS warning, indicating that your BIOS cannot find a bootable disk. The most likely cause of this is a messed up boot order in the BIOS.
That being said, there's a big difference between something running, and something working well. That Optiplex is well below the recommended hardware, and with only 4GB RAM you have have the required memory. Will it run? Probably. Will you run in to problems because of your hardware? Again, probably.
It sounds like you are trying to install on a hard drive. That is generally a bad idea, because it takes up the entire drive, leaving no space for storage. Most people want a USB or small SSD as their boot device.