Hi,
The reason for the error is that the release code 10 is not affected with release and hence the pre requisite for release code 20 is not full filled.
While simulating, first select the release code 10 (It will be shown as "Release Possible") and click on "Simulate Release". Now for release code 10, it will be shown as release affected. Also, the release code 20 will be shown as release pre requisite full filled. Now, select the release code 20 and click on simulate release.
Regards,
AKPT