Start editing in ArcMap encounters one or more layers with warnings. If you continue, you may not be able to edit some layers. The Warning box

Errors that may occur after you start editing:

If ArcMap encounters a problem starting an edit session on the selected data, a dialog box pops up to provide additional information. You may receive an error, warning, or informational message.

• When an error occurs, the user cannot start any edit sessions. You can edit the data only if the problem is resolved. You receive an error message if there are no editable data sources, licensing issues (for example, if you try to edit certain types of geodatabase features while using ArcGIS for Desktop Basic), or if some layers exist in the basemap layer.
• After a warning appears, you can still start an edit session, but you cannot edit some items in the map. Before you can edit, you need to resolve the problem with that particular layer or table.
• Informational messages provide more advice on how to improve performance when editing. You do not need to resolve the issue before you edit the layer or table, but it is strongly recommended that you do so. For example, if the data and data frame you are editing are in a different coordinate system, an informational message informs you of the situation. If your data uses a coordinate system that differs from the data frame in which the data resides, you can still edit the data. However, when you perform certain editing tasks, you may have unexpected alignment or accuracy issues.

The above is reproduced from the official website: about edit session

When you recently started an edit session in ArcMap by clicking the Editor menu on the Editor toolbar or right-clicking a layer in the table of contents, the start editing encounters one or more layers with warnings. If you continue, you may not be able to edit some layers. warning boxes, such as:

Follow the prompts: Double-click the error to see additional information:

ArcGIS Help 10.3 Shows-- info: The spatial reference does not match the data frame.

The spatial reference of the layer does not match the projection of the data frame.

Solution: Make sure that the projection information for the layer matches the other layers and matches the data frame. This may indicate that you need to re-project the layer to match the data frame, or you need to change the projection of the data frame to match the layer.

More information: This warning message appears when the projection of the data frame does not match the layer in the map. This warning message is typically displayed after adding a layer from multiple workspaces, because the data frame uses the spatial reference of the first layer added to the map. Before you can display this warning message, you may not be informed that some data is currently being projected dynamically.

You can still edit the layers in the map, note that you can create data or modify inaccurate data after saving. For an example of editing an effect when projecting dynamically, see about editing data in different projections (dynamic projection).

Start editing in ArcMap encounters one or more layers with warnings. If you continue, you may not be able to edit some layers. The Warning box