When you click on Imagery > ESRI Layer, verify the REST location is pointing to the correct server path.  If you do not see anything listed under the 'Map List', select 'Modify' and verify the URL path is exactly the same as the link below.  The path needs to have the HTTPS and not just HTTP as the ESRI imagery was changed to a secure server.  You should see all the available Maps once you are pointing to the correct server path.

https://services.arcgisonline.com/ArcGIS/rest/services

This video demonstrates how to set the correct REST Server path to load ESRI Layers OpenSite SiteOps.

Additional REST Server Information

Accessing the REST Server List

Modifying the REST Server List