Turns out it was a setting changed because as SAS card was booting first and forgot to change the boot back. So the setting was at BIOS - Integrated Devices - Slot Disablement. The slot was set to boot drive disabled. It's boot now, sucessfully.
Have a new R640 with a single SSD on a BOSS-S1 controller set as AHCI controller in slot 1. No other storage or controllers. Anyone know how why a poweredge R640 can't boot the OS from this?   fresh Windows Server 2019 Datacenter install that won't boot after install completes “boot failed: windows boot manager”  bios shows: For the hard drive Boot sequence shows "unavailable:windows boot manager"   Boot set to UEFI  
R640 single SSD with BOSS-S1 “boot failed: windows boot manager”