The layer (<layer_name>) is not editable

Problem:

While creating on an MDB following error message is shown:

The layer (<layer_name>) is not editable !

Cause:

Probably a unique index was created on a field in the related feature class or any other feature class within the FDS. Basically this is allowed on a MDB, but exludes editing of the features.

https://desktop.arcgis.com/en/arcmap/latest/manage-data/tables/creating-attribute-indexes.htm

Solution:

Drop related indexes in ArcCatalog and re-create them non-unique.