No Managed Server available message when trying to add an applicattion monitor

Hi,
When I try to add an OAS monitor to an already monitored server with snmp, I get this:
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Select a Managed Server

Least loaded one from all Managed Servers
Least loaded one from a Managed Server Group
Specific Managed Server

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But I dont know for which server is the APP feature asking.... I am monitoring the desired server with snmp with no problem, but I want to add the Applicattion monitor too, and I couldn´t :-(

Message: No Managed Server available

Thanks in advance
Carlos