13th Generation of servers does not support Ubuntu that is why you are not getting an option for Ubuntu for R430. If you want to update BIOS on R430 you can use iDRAC which is independent of operating system installed on the server. You can refer below link for details on same [URL]  For updating BIOS using iDRAC you need to download BIOS package with "Update Package for MS Windows 64-Bit." format using below link [URL] 
Hi Dell support team, I have R430 and R440 PE servers are recently on BIOS updates. They are all running Ubuntu 18.04. R440 is working fine with DUP available here: [URL] and is Ubuntu 18 supported. I'm happy with the DUP. However it looks like there's no Ubuntu support for R430, could you please guide me to the right place/package for the update?   Thanks! Freddy    
No support for R430 BIOS update with Ubuntu18.04?