Metrics cannot be paired with dimensions of the same scope.
Dimensions are attributes of your data. For example, the dimension City indicates the city, for example, “Paris” or “New York”, from which a session originates. The dimension Page indicates the URL of a page that is viewed.
Metrics are quantitative measurements. The metric Sessions is the total number of sessions. The metric Pages/Session is the average number of pages viewed per session.
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.
Read more here: https://support.google.com/analytics/answer/1033861