Hi Thomas,
2016 SP2 CU6 (released 20th March) is already known in SQL Monitor, and is currently the version SQL Monitor recommends for 2016 servers. You can see this for example for the 2016 servers at https://monitor.red-gate.com/Estate/Versions. SQL Monitor should download the latest version definitions every 6 hours, but if your SQL Monitor base monitoring service can't connect to www.red-gate.com, it won't be able to do this. In that case you can update the versions list manually using the steps at the bottom of https://documentation.red-gate.com/sm9/estate/versioning-and-patching. If that still doesn't work I'd suggest contacting Redgate's support team. I'll go ahead and mark this suggestion as completed for now, since that version is available in the latest definitions.