When try to delete an Application User, we are hitting Event Journaling error as below.
If we go to download the log file, you can see exactly what is the details error.
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/"><s:Body><s:Fault><faultcode>s:Client</faultcode><faultstring xml:lang="en-US">The 'Delete' method does not support entities of type 'systemuser'.</faultstring><detail><OrganizationServiceFault xmlns="http://schemas.microsoft.com/xrm/2011/Contracts" xmlns:i="http://www.w3.org/2001/XMLSchema-instance"><ActivityId>e018ef03-8ad4-4e65-8b2c-d0b655331a4f</ActivityId><ErrorCode>-2147219456</ErrorCode><ErrorDetails xmlns:a="http://schemas.datacontract.org/2004/07/System.Collections.Generic"><KeyValuePairOfstringanyType><a:key>ApiExceptionSourceKey</a:key><a:value xmlns:b="http://www.w3.org/2001/XMLSchema" i:type="b:string">Api/PrePipeline</a:value></KeyValuePairOfstringanyType><KeyValuePairOfstringanyType><a:key>ApiOriginalExceptionKey</a:key><a:value xmlns:b="http://www.w3.org/2001/XMLSchema" i:type="b:string">Microsoft.Crm.CrmException: The 'Delete' method does not support entities of type 'systemuser'.
at Microsoft.Crm.Extensibility.InternalMessageDispatcher.Execute(PipelineExecutionContext context)
at Microsoft.Crm.Extensibility.ExternalMessageDispatcher.ExecuteInternal(IInProcessOrganizationServiceFactory serviceFactory, IPlatformMessageDispatcherFactory dispatcherFactory, Guid callerRegardingObjectId, Int32 invocationSource, Version endpointVersion, OrganizationRequest request, OrganizationInternalRequest organizationInternalRequest)
at Microsoft.Crm.Extensibility.OrganizationSdkServiceInternal.DispatchRequest(OrganizationRequest request, OrganizationInternalRequest organizationInternalRequest, SdkResponseDescription responseDescription, SdkPerformanceCounterContext sdkPerformanceCounterContext)
at Microsoft.Crm.Extensibility.OrganizationSdkServiceInternal.ExecuteRequestRequestWithInstrumentation(OrganizationRequest request, OrganizationInternalRequest organizationInternalRequest)
at Microsoft.Crm.Extensibility.OrganizationSdkServiceInternal.ExecuteRequestRequestWithInstrumentationOperation(OrganizationRequest request, OrganizationInternalRequest organizationInternalRequest)
at Microsoft.Crm.Extensibility.OrganizationSdkServiceInternal.<>c__DisplayClass20_0.<ExecuteRequest>b__0()</a:value></KeyValuePairOfstringanyType><KeyValuePairOfstringanyType><a:key>ApiStepKey</a:key><a:value xmlns:b="http://schemas.microsoft.com/2003/10/Serialization/" i:type="b:guid">00000000-0000-0000-0000-000000000000</a:value></KeyValuePairOfstringanyType><KeyValuePairOfstringanyType><a:key>ApiDepthKey</a:key><a:value xmlns:b="http://www.w3.org/2001/XMLSchema" i:type="b:int">0</a:value></KeyValuePairOfstringanyType><KeyValuePairOfstringanyType><a:key>ApiActivityIdKey</a:key><a:value xmlns:b="http://schemas.microsoft.com/2003/10/Serialization/" i:type="b:guid">e018ef03-8ad4-4e65-8b2c-d0b655331a4f</a:value></KeyValuePairOfstringanyType><KeyValuePairOfstringanyType><a:key>ApiPluginSolutionNameKey</a:key><a:value xmlns:b="http://www.w3.org/2001/XMLSchema" i:type="b:string">System</a:value></KeyValuePairOfstringanyType><KeyValuePairOfstringanyType><a:key>ApiStepSolutionNameKey</a:key><a:value xmlns:b="http://www.w3.org/2001/XMLSchema" i:type="b:string">System</a:value></KeyValuePairOfstringanyType><KeyValuePairOfstringanyType><a:key>ApiExceptionCategory</a:key><a:value xmlns:b="http://www.w3.org/2001/XMLSchema" i:type="b:string">ClientError</a:value></KeyValuePairOfstringanyType><KeyValuePairOfstringanyType><a:key>ApiExceptionMesageName</a:key><a:value xmlns:b="http://www.w3.org/2001/XMLSchema" i:type="b:string">SdkEntityDoesNotSupportMessage</a:value></KeyValuePairOfstringanyType><KeyValuePairOfstringanyType><a:key>ApiExceptionHttpStatusCode</a:key><a:value xmlns:b="http://www.w3.org/2001/XMLSchema" i:type="b:int">400</a:value></KeyValuePairOfstringanyType></ErrorDetails><HelpLink>http://go.microsoft.com/fwlink/?LinkID=398563&error=Microsoft.Crm.CrmException%3a80040800&client=platform</HelpLink><Message>The 'Delete' method does not support entities of type 'systemuser'.</Message><Timestamp>2020-11-09T06:05:51.3174301Z</Timestamp><ExceptionRetriable>false</ExceptionRetriable><ExceptionSource i:nil="true"/><InnerFault i:nil="true"/><OriginalException i:nil="true"/><TraceText i:nil="true"/></OrganizationServiceFault></detail></s:Fault></s:Body></s:Envelope>User is not able to delete, we can only disable the user, but I am not sure why there is a Delete button in D365 online which create confusions.