As mentioned in the following Blog post, Microsoft Dynamics 365 (online) version 9.0 bocks older versions of TLS (1.0 and 1.1). TLS 1.2 (or better) is now required. If you are using version 8.x of Microsoft Dynamics 365 for Outlook, an update is required to connect to a version 9.0 instance. Please refer to the following articles for additional details:

Updates coming to Dynamics 365 Customer Engagement connection security

https://blogs.msdn.microsoft.com/crm/2017/09/28/updates-coming-to-dynamics-365-customer-engagement-connection-security/ 

"There is a problem communicating with the Microsoft Dynamics 365 server" error occurs when configuring Microsoft Dynamics 365 for Outlook

https://support.microsoft.com/help/4073398

As mentioned in the KB, if you expand the Details section of the error, the following additional details are shown:

Metadata contains a reference that cannot be resolved: 'https://crmorgname.api.crm.dynamics.com/XRMServices/2011/Organization.svc?wsdl&sdkversion=8.2'.    at System.ServiceModel.Description.MetadataExchangeClient.MetadataRetriever.Retrieve(TimeoutHelper timeoutHelper)
   at System.ServiceModel.Description.MetadataExchangeClient.ResolveNext(ResolveCallState resolveCallState)
   at System.ServiceModel.Description.MetadataExchangeClient.GetMetadata(MetadataRetriever retriever)
   at System.ServiceModel.Description.MetadataExchangeClient.GetMetadata(Uri address, MetadataExchangeClientMode mode)
   at Microsoft.Xrm.Sdk.Client.ServiceMetadataUtility.RetrieveServiceEndpointMetadata(Type contractType, Uri serviceUri, Boolean checkForSecondary)
   at Microsoft.Xrm.Sdk.Client.ServiceConfiguration`1..ctor(Uri serviceUri, Boolean checkForSecondary)
   at Microsoft.Xrm.Sdk.Client.OrganizationServiceConfiguration..ctor(Uri serviceUri, Boolean enableProxyTypes, Assembly assembly)
   at Microsoft.Xrm.Sdk.Client.ServiceConfigurationFactory.CreateConfiguration[TService](Uri serviceUri, Boolean enableProxyTypes, Assembly assembly)
   at Microsoft.Xrm.Sdk.Client.ServiceConfigurationFactory.CreateConfiguration[TService](Uri serviceUri)
   at Microsoft.Crm.Outlook.ClientAuth.ClientAuthProvidersFactory`1.DiscoverAuthUsingServiceMetadata(Uri endPoint, Uri webEndPoint)
   at Microsoft.Crm.Outlook.ClientAuth.ClientAuthProvidersFactory`1.GetAuthProviderForDeployment(Uri endPoint, Uri webEndPoint)
   at Microsoft.Crm.Application.Outlook.Config.DeploymentsInfo`1.GetAuthenticatedProvider(OrganizationDetail orgDetail, Control parentWindow)
   at Microsoft.Crm.Application.Outlook.Config.ServerForm.LoadDataToServerInfo()
   at Microsoft.Crm.Application.Outlook.Config.ServerForm.<InitializeBackgroundWorkers>b__3(Object sender, DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)