You attempted to add a value to a field that is inappropriate for that value. For instance, a text value with a width of 10 cannot be merged into a field that has a width of 5. The record shown with this warning will be omitted from the output. This warning also results from adding a null value to a field that does not support null values. The operation will continue, but the record provided in the message will be excluded from the output. Not all data types allow fields that support null values; for instance, shapefiles and dBASE tables do not, but geodatabase feature classes and tables do. Even when they're supported by the data format, null values can be restricted from a field by setting Allow Null Values to No when the field is created. An example of when this message occurs is if you're appending one dataset to another, in which an input dataset includes a field that supports nulls (and contains nulls) and is mapped to a field in the target dataset that does not support nulls.
When possible, use field properties that are inclusive of any values that may potentially be added. For instance, when merging or appending datasets, make sure that the output field supports the largest of the text field widths. When dealing with nulls, the Preserve Nulls environment can be turned on or off to control whether nulls are preserved in the output. If turned off, the tool used will attempt to add null values to a field or format that will not accept them.
Field properties for the target dataset cannot be changed when using field mappings in the Append tool. Use the environment discussed above or change the field property prior to using the tool.