Dimensions won't appear as an option under a Dimensional Metric until that Dimension has been associated with a Metric value.
For example, "Campaign A" will not show up under the Dimensional Metric "Impressions by Campaign" until the first Impression for Campaign A is tracked. This means that if the first impression for Campaign A was tracked this month, Campaign A will not show as a dimension when "Last Month" is selected as the Date Range.
Similarly, a new page on your website will not show up as a Dimension option under the Dimensional Metric "Pageviews by Page" until the first Pageview is recorded.
This is also the case for the HubSpot CRM Dimensional Metric "Closed Won Deals by Owner." An Owner will not show up until they have at least 1 Closed Won Deal associated with them in HubSpot.