You are sharing a web map or web layer and you have chosen to reference registered data. However, the data source referenced by this layer, or data that is joined or related to this layer, is not registered with the federated server. The data will be copied to the server.
Solution
Allow the data to be copied to the server.
Do nothing. The source data, including all joined or related data, will be copied to the federated server.
Register the data with the server.
Hover over the analyzer message. Click the Options button
that appears and click
Register Data Source With Server. On the Add data store dialog box, complete the required information, and click Create. Note:
If you do not have the privilege to register data stores in your ArcGIS organization, a message to contact your organization administrator appears. Your administrator can assign you the privilege to register data stores or can register the data store for you.
The option to specify a server database connection does not support SQL Server databases using Azure Active Directory Universal with MFA. For more information, see Azure Active Directory options.
If the server is federated with the portal, view the registered data stores and add a new data store.
Hover over the analyzer message. Click the Options button
that appears and click
Open Manage Data Stores Pane. Click the Add button 
and choose the type of data store the web layer will reference data from. On the Add data store dialog box, complete the required information, and click Create.
More information
Registering data with the server helps ensure that the server has access to the layer's data source. For more information on registering data, see Manage registered data stores.
You may encounter this message even though the source enterprise geodatabase or enterprise database has already been registered with the federated server. The reason for this is that the connection properties (user name, password, version, and so on) used by a layer or stand-alone table referencing the source enterprise geodatabase do not match the connection properties of the enterprise geodatabase registered with the server. Registration is required for each separate connection.
For example, suppose you have an enterprise geodatabase named DB1 that has two versions: default and version 1. You only register the connection to DB1 using the default version. If you attempt to share a map containing layers or stand-alone tables pointing to version 1, and you have not registered this connection to DB1, you will encounter this message. This would also occur if you have different connections to the geodatabase using different user names.