Troubleshooting “Nodemanager Associated with the machine is not reachable”

While trying to start Managed servers from the Administration Console, you may receive the below error

For server soa_server1, the Node Manager associated with machine Machine1 is not reachable. 
All of the servers selected are currently in a state which is incompatible with this operation or are not associated with a running Node Manager or you are not authorized to perform the action requested. No action will be performed.

To fix this issue.

1) Check is the nodemanager is running at OS level using command

$ps -ef|grep -i nodemanager 

2) Verify the nodemanager logfile has no errors

3)Check if the managed server is properly associated with the machine

4) Check the nodemanager configuration(credentials/ Hostname/Port number) details are correctly updated in Console

In the Change Center of the Administration Console, click Lock & Edit (see Use the Change Center).
In the left pane of the Console, expand Environment and select Machines.
Select the machine for which you want to configure Node Manager.
Select Configuration > Node Manager.
In the Type field, select the Node Manager type from the drop-down list.
For more information on Node Manager types, see Node Manager Administrator’s Guide.
Note: The values you provide here must correspond to the values you used to configured the Node Manager instance.
In the Listen Address field, enter the DNS name or IP address upon which Node Manager listens.
If you identify the Listen Address by IP address, you must disable Host Name Verification on Administration Servers that will access Node Manager. For more information and instructions, see Using Host Name Verification in Managing WebLogic Security.
Enter a value in the Listen Port field. This is the port where Node Manager listens for incoming requests.
If you have set the Type field to SSH or RSH, you should specify values in the Node Manager Home and Shell Command fields.
For more information on configuring Node Manager using SSH or RSH, see Node Manager Administrator’s Guide.
Click Save.
To activate these changes, in the Change Center of the Administration Console, click Activate Changes. 
Not all changes take effect immediately—some require a restart (see Use the Change Center).