If a project has a nested multi-module structure, you can't use the "Release" button in Continuum. The error is:
Cannot release two or more parent projects in the same project group at the same time.
Workaround: Find the top-level parent pom within the group, and click the release icon to the right of it
While it would be an error to try to release two separate hierarchies at the same time, ontinuum Release needs to understand how to find the "top level" parent pom in a group containing projects within a single hierarchy.