I've upgraded from 18.04 in the past, but via intermediary LTS releases (i.e. 20.04). Going all in one step is not possible IMO.

Also note that Ubuntu LTS releases only do upgrades after the first point release of a new version. You won't be upgraded to 21.10 until it hits 21.10.1.

[edit] Apparently the holdout until point release .1 only applies to LTS versions:

> 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. [https://ubuntu.com/server/docs/upgrade-introduction]