You have upgraded your Enforce Server to 15.7 and your detection servers are still running 15.5.

OR

You have setup a new version 15.1 Enforce server and you are trying to move your existing 14.6.x Detection servers from the preexisting 14.6 (old) environment to this new Enforce server but when you have added the server the status shows as Unknown.

When you click on the Detection server page in the Enforce console you notice the following message: 

!This server requires upgrading. Until upgraded, only minimal functionality will be available and a server restart will disable all functionality.

In the server events you see the the following Event Code 2322 as follows: 

Version 15.5.x  Detection server is not fully supported or operational with a version 15.7.x Enforce server.

Version 14.6.x Detection server is not fully supported or operational with a version 15.1 Enforce server.

Going from version 14.6 to 15.1 while skipping 15.0 is a jump between major versions.

You must upgrade your detection servers to the same version as the Enforce server to be fully supported or operational.