You can combine a metric X with a dimension Y in Google Analytics
- A) if X and Y have the same scope
- B) as long as sampling is not required
- C) if X and Y are in the same channel grouping
- D) if X and Y have been precalculated together in an aggregate table
- E) If X and Y have the same campaign
Explanation: Not every metric can be combined with every dimension. Each dimension and metric has a scope: user-level, session-level, or hit-level. In most cases, it only makes sense to combine dimensions and metrics that share the same scope. For example, Sessions is a session-based metric so it can only be used with session-level dimensions like Source or City. It would not be logical to combine Sessions with a hit-level dimension like Page.
Read more here: //support.google.com/analytics/answer/1033861?hl=en