The Report Designer cannot combine metrics in endless cuts since there are limitations and some metrics are connected in more granular ways than others. For example, we can break out our ad serving events into a ton of details about which ad campaigns, ad sets, and ad engines were used. But we cannot break down videos the same way.
As you start selecting dimensions and metrics, your choice is automatically limited in the report designer. Instead of throwing an error message, we just give you metrics and dimensions that are feasible. If you pick 'Errors' but no longer see 'Ad campaign id', that's because you cannot break down errors that way.
If you want to see all the available options, just start removing dimensions and metrics you have already selected.