I'm using Azure DevOps pipelines and for 'builds' I can define 'Hosted Ubuntu' agent (in my pipelines.yml file). But for 'releases' there's no way to edit yaml just the UI. The agent pool drop down list shows 'Default (no agents)'. I assume my problem is probably related to project permissions but haven't figured out what controls the visibility. On another account I see all the hosted agent options available.
Check that you are part of the group(s) assigned as User to the security of the app pool in question.
If this security operates like other Allow, Deny, NotSet permissions, then your occupation in a group that has less than User permission will revoke that permission, but I haven't seen any documentation about this.
Yaml usage in the formal release pipeline hub is not supported. If you want to use yaml for your CD release, you're going to need to enable the new multi-stage pipelines experience.
This allows you to create stages in your .yml file that relate to deployment functions in addition to the classic build functions.
I believe this removes your release tracking from the release hub and integrates it directly into the build hub.
This is still a new concept for Azure DevOps, as is the documentation and writing about it. I found this article to be helpful.
Because this new feature seems to be centered around AKS (or other Kubernetes services) and Docker, I'm not sure if this will help you by allowing you to specify the host in yaml, but you mentioned "release" and "yaml" in the same sentence, so I thought I would address that.
dev.azure.com/{organization}/{project}/_settings/agentqueuessettings? – Josh Gust