You are sharing a web feature layer or a scene layer with an associated feature layer. The feature layer contains data that is z-aware but no default z-value is defined. This means that you cannot edit the web layer in Map Viewer Classic or in other clients that do not allow you to set z-values. (You are able to edit the web layer in clients such as ArcGIS Pro that allow you to set z-values.)
Set a default z-value for features.
Hover over the analyzer message. Click the Options button that appears and click Show Feature Properties. In the Share As Web Layer pane, on the Configuration tab, check the Apply default to features with z-values box and type a default value to use when features are inserted or updated.
Setting a default z-value ensures that features can be inserted or updated in clients that do not support setting a z-value.