When updating your SQL server (Standalone) with a Cumulative Update (CU) you might check the error in the Rule Check Result.

Rule "Not clustered or the cluster service is up and online." failed.

The machine is clustered, but the cluster is not online or cannot be access from one of its nodes. To continue determine why the cluster is not online and rerun setup instead of rerunning the rule since the rule can no longer detect a cluster environment correctly.

This is a very strange message, especially since the SQL server is a stand alone server and there is no cluster. After a restart of the server and re-trying the update, the problems still persists.

After a lot of investigation, this issue is nothing related to SQL, but WMI. This can be verified by opening the WMI Management using the command WMIMGMT.MSC, right-click on WMI Control (Local) and click on Properties.

If your entries show Invalid Class, as the above screenshot, we need to re-compile the MOF files using the following command from a command prompt as Administrator.

mofcomp C:\Windows\System32\WBEM\cimwin32.mof

The result should be as below

After this, if you rerun the WMIMGMT.MSC and the same process, the WMI should look like this.

You can now re-run the SQL update and it will update with no issues, thus solving the problem permanently.