While running the sudo do-release-upgrade command you are getting the following issue:
There is no development version of an LTS available. To upgrade to the latest non-LTS development release
The above warning could be because of the LTS version which you are about to update has still not released as first point release. You will have to check release plan from their official docs.
Further, from https://ubuntu.com/server/docs/upgrade-introduction resource
Upgrades from one LTS to the next LTS release are only available after the first point release. For example, Ubuntu 18.04 LTS will only upgrade to Ubuntu 20.04 LTS after the 20.04.1 point release.