Testing monitor fails if monitor exist?

Hi!
I recently started working with F5 and tmsh, and I'm having problems with a monitor. 

 - The monitor and node are in the same partition.
 - The node is a fqdn node with 2 ephemeral ips (auto populate).
 - The monitor finds the URL Available, but the IPs are Offline. I already checked with curl and with nc the IPs (as described in https://support.f5.com/csp/article/K81239024) and they respond 200 to the "Send String"

The only test I'm missing is the monitor configuration test (https://support.f5.com/csp/article/K60677941 and https://support.f5.com/csp/article/K13522220) but both gives an exception: "The requested monitor instance ... already exists". 
Is this an F5 bug or i'm missing something? Are other ways to test and understand why the monitor is failing to recognize the IP is Available?  

Edit: The original problem was solved. The auto populate node gets a ICMP monitor by default, even when you are setting a HTTP monitor. Behind the node was kubernetes and it didn't have ping enabled, so it failed. You can change the type of monitor in the Properties tab of the fqdn node, using "Node Specific" in "Health Monitors"

The original problem is still unsolved. Why the test to check the configuration of the monitor fails with "The requested monitor instance ... already exists"?