Why isn't an upgrade to 20.04 from 18.04 available yet?

See https://discourse.ubuntu.com/t/focal-fossa-20-04-1-lts-point-release-status-tracking/17604 for the current status.

On 25 September 2020, the final upgrade-blocking bug was closed.

If you are reading this on 25-27 September, just be patient while your local mirror catches up.

sudo do-release-upgrade -d

Will get you safely upgraded today, but please verify before upgrading that you are not affected by the blocking bugs mentioned above by user535733.

If you have a 32-bit installation then you will never be offered an upgrade to 20.04 because it is not supported anymore. If this is the case then your only way is to reinstall with the 64-bit version ;-(

How to check quickly: In a terminal enter

uname -m

If this gives you i686 and not x86_64 then you have 32-bit installation.

Hope that helps