With some help from UiPath I figured it out. Thanks @Robert_Waldinger !
First of to see where NLog is having issues one can add a Debug Flag to help identify the issue
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" internalLogLevel="Debug" internalLogFile="C:\tmp\NLOG_DEBUG.log">
This way I could see the the ElasticSearch assembly could not be loaded
After some trial and error I figured out that Elasticsearch.Net.dll, Nest.dll and Log.Targets.ElasticSearch.dll are missing from C:\Program Files\UiPath\Studio\net461
I downloaded the assembly using the Nuget CLI NuGet Gallery | Downloads
.\nuget.exe install NLog.Targets.ElasticSearch -Version 7.7.0 -DirectDownload -Framework net461 -Verbosity normal -Source https://api.nuget.org/v3/index.json -OutputDirectory .\download_folder -NonInteractive
fetched the three .dll from their respective folders. For example:
and copied them to C:\Program Files\UiPath\Studio\net461
Now the target was working and through the debug errors I could find out that I was using the wrong format for the APIKey.
When producing a Key in ElasticCloud one gets the Keys in this format.
<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<extensions>
<add assembly="NLog.Targets.ElasticSearch"/>
</extensions>
<variable name="WorkflowLoggingDirectory" value="${specialfolder:folder=LocalApplicationData}/UiPath/Logs" />
<targets>
<target type="File" name="WorkflowLogFiles" fileName="${WorkflowLoggingDirectory}/${shortdate}_Execution.log" layout="${time} --##-- {level} --##-- ${message}" keepFileOpen="true" openFileCacheTimeout="5" concurrentWrites="true" encoding="utf-8" writeBom="true" />
<target xsi:type="ElasticSearch" name="robotElastic" uri="https://xxxxx.cloud.es.io"
cloudId="xxxxxxx"
apiKeyId="xxxxx"
apiKey="xxxxxx"
index="robot-${date:format=yyyy.MM}"
documentType=""
includeDefaultFields="true"
includeAllProperties="true"
layout="${message}"
requireAuth="false"
/>
</targets>
<rules>
<logger name="WorkflowLogging" minlevel="Info" writeTo="robotElastic,WorkflowLogFiles" final="true" />
</rules>
</nlog>
And It finally works 
