ArcGIS Online: Unable to add layer. Feature layer must be created with URL or source

Error Message

When attempting to open a web map published from ArcGIS Pro to ArcGIS Online in Map Viewer, the following error message is returned:

Error: 
Unable to add layer <layer name>
Feature layer must be created with either a url or a source

Cause

• The layer name contains unsupported special characters such as '|'.
• The layer’s primary symbology relies on one or more invalid fields. When the layer’s symbology is configured in the ArcGIS Pro Symbology pane, the following warning message is returned:

Warning:
Primary symbology relies on one or more fields that are invalid.

Solution or Workaround

Remove the special characters from the layer’s name in ArcGIS Pro

If the layer name contains special characters, apply the following steps to remove the special characters and rename the layer.

1. Open the project in ArcGIS Pro.
2. In the Contents pane, right-click the feature layer and select Properties.
3. In the Layer Properties window, click the General tab.
4. In the Name box, remove the special character or rename the layer without special characters and click OK.
5. Publish the map to ArcGIS Online. Refer to ArcGIS Pro: Share a web map for instructions.

Populate a valid field in ArcGIS Pro

To resolve the error returned in Map Viewer and the warning message displayed in the ArcGIS Pro Symbology pane, refer to Problem: Importing symbology returns a warning message in ArcGIS Pro for more information and instructions, and republish the map.