Thanks Andrey for replay,
I have system administrator role in online CRM and have all permissions for both account and contact entities and enabled plugin trace log in CRM and got following error:
Unhandled exception:
Exception type: System.ServiceModel.FaultException`1[Microsoft.Xrm.Sdk.OrganizationServiceFault]
Message: An unexpected error occurred from ISV code. (ErrorType = ClientError) Unexpected exception from plug-in (Execute): Plugins.AssignAutoNumberPlugin: System.NullReferenceException: Object reference not set to an instance of an object.Detail:
<OrganizationServiceFault xmlns:i="www.w3.org/.../XMLSchema-instance" xmlns="schemas.microsoft.com/.../Contracts">
<ActivityId>e8fb57d4-0616-4d16-a478-fd590d07805f</ActivityId>
<ErrorCode>-2147220956</ErrorCode>
<ErrorDetails xmlns:d2p1="schemas.datacontract.org/.../System.Collections.Generic" />
<HelpLink i:nil="true" />
<Message>An unexpected error occurred from ISV code. (ErrorType = ClientError) Unexpected exception from plug-in (Execute): Plugins.AssignAutoNumberPlugin: System.NullReferenceException: Object reference not set to an instance of an object.</Message>
<Timestamp>2019-10-16T09:42:20.2082345Z</Timestamp>
<ExceptionRetriable>false</ExceptionRetriable>
<ExceptionSource>PluginExecution</ExceptionSource>
<InnerFault i:nil="true" />
<OriginalException>System.NullReferenceException
at Plugins.AssignAutoNumberPlugin.Execute(IServiceProvider serviceProvider)
at Microsoft.Crm.Sandbox.SandboxAppDomainHelper.<>c__DisplayClass14_3.<Execute>b__0()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at Microsoft.Crm.Sandbox.SandboxAppDomainHelper.Execute(IOrganizationServiceFactory organizationServiceFactory, Dictionary`2 sandboxServices, String pluginTypeName, String pluginConfiguration, String pluginSecureConfig, IPluginExecutionContext requestContext, Boolean enablePluginStackTrace, Boolean chaosFailAppDomain, String crashOnPluginExceptionMessage)
at Microsoft.Crm.Sandbox.SandboxAppDomainHelper.Execute(IOrganizationServiceFactory organizationServiceFactory, Dictionary`2 sandboxServices, String pluginTypeName, String pluginConfiguration, String pluginSecureConfig, IPluginExecutionContext requestContext, Boolean enablePluginStackTrace, Boolean chaosFailAppDomain, String crashOnPluginExceptionMessage)
at Microsoft.Crm.Sandbox.SandboxWorker.<>c__DisplayClass3_0.<Execute>b__0()</OriginalException>
<TraceText i:nil="true" />
</OrganizationServiceFault>