1. You installed Acronis Cyber Backup 12.5 Update 4 (build 13400 and higher),
2. When you navigate to Settings -> Agents -> Details, you see that Agent is shown offline since year 1754 (exact date depends on your environment). You cannot run a backup plan on this machine or apply new backup plans to it.
Issue in the product.
The issue has been fixed in Update 5 (Build 16180), please update to the latest build.
If product update did not resolve the issue, collect Acronis System Information from the Management Server and affected Agent machine and contact Acronis Support.