When creating an initial job that manually triggers a downstream job in the post-build actions, the downstream job cannot be started using the Delivery Pipeline view because there is no "play" button visible/available after the initial job finishes. However, when using the Build-Pipeline-View, the downstream job is able to be triggered manually. There are no errors showing up in the logs that I see.
To recreate, use the versions listed in the environment section, and create two freestyle projects. In the first project add a post-build action to "Build other projects (manual step)" and set the second project as the "Downsteam Project Name." Create a Delivery pipeline view and set the initial job as the first project you created. Build project 1 and view the delivery pipeline. The second job never has a play button to run.
To add context to the provided image attachment, you can see there are three jobs test1, test2, and test3. Test1 and Test2 are setup as described above, but Test3 was added via "Build other projects" in Test1's post-build actions. I did this to show automatic build triggering is working but, not manual as you can see with Test2.