Not all data in Google Analytics is equal. Or more precisely – it cannot be combined together.
Avinash Kaushik had dealt with this issue in one of his famous articles. He explained that some metrics were calculated on a hit-level and some on a session-level. He uses wonderful visualisations to understand the difference, therefore I just refer to his article.
The point is that it is not possible to create a custom report using Google Analytics data and combine any type of data. The resulting report would not result in any meaningful insights, as your data would not be correct.
However, it is not always easy to say, if a particular dimension or metric is session-level or hit-level.
A good way how to make sure I can combine these metrics is to use a Dimensions & Metrics Explorer by Google Analytics.
Among other things, it allows you to check metrics and dimensions.
If it is possible to select a checkbox with a specific set of metrics or dimensions, the tool suggests that it is possible to combine these metrics and dimensions.
The guide says: „Select a dimension or metric checkbox to see all the other values that can be combined in the same query.”
In this way, you may see that it is possible to use only some of the metrics and dimensions.