Web maps require a basemap. The basemap is used to define the spatial reference of the web map and the tiling scheme that all cached layers must follow. This ensures that data lines up and displays correctly across all client applications. Because the basemap's coordinate system is used by the web map, before you share your map, set the coordinate system of your map to match the coordinate system of the basemap.
You can add basemaps in ArcGIS Pro from the Basemap gallery on the Map tab. Depending on how your active portal is configured, the gallery can include the default Esri basemaps, maps shared to your organization's basemap gallery group, and basemaps you have added to your project in ArcGIS Pro.
For more information on authoring a custom basemap, see Author a custom basemap.