External Table: VARCHAR in Create but CHAR in Access Parameters
Datatypes inside the database, and used by SQL*Loader or the ORACLE_LOADER driver are different.

SQL*Loader does not know the VARCHAR2 datatype. It has a VARCHAR type, but that type is nonportable, meaning it is platform dependent.

SQL*Loader Datatypes


  VARCHAR fields can be loaded with correct results only between systems
  where a SHORT data field INT has the same length in bytes. If the byte
  order is different between the systems, or if the VARCHAR field
  contains data in the UTF16 character set, then use the appropriate
  technique to indicate the byte order of the length subfield and of the
  data. The byte order of the data is only an issue for the UTF16
  character set.


The CHAR type of SQL*Loader is portable.


  Portable Datatypes
  
  The portable datatypes are grouped into value
  datatypes and length-value datatypes. The portable value datatypes are
  as follows:
  
  CHAR
  
  Datetime and Interval
  
  GRAPHIC
  
  GRAPHIC EXTERNAL
  
  Numeric EXTERNAL (INTEGER, FLOAT, DECIMAL, ZONED)
  
  RAW
  
  The portable length-value datatypes are as follows:
  
  VARCHARC
  
  VARRAWC


Using a non-portable type you may or may not be able to load the data on a different platform:

Loading Data Across Different Platforms


  When a data file created on one platform is to be loaded on a
  different platform, the data must be written in a form that the target
  system can read. For example, if the source system has a native,
  floating-point representation that uses 16 bytes, and the target
  system's floating-point numbers are 12 bytes, then the target system
  cannot directly read data generated on the source system.
  
  The best solution is to load data across an Oracle Net database link,
  taking advantage of the automatic conversion of datatypes. This is the
  recommended approach, whenever feasible, and means that SQL*Loader
  must be run on the source system.
  
  Problems with interplatform loads typically occur with native
  datatypes. In some situations, it is possible to avoid problems by
  lengthening a field by padding it with zeros, or to read only part of
  the field to shorten it (for example, when an 8-byte integer is to be
  read on a system that uses 4-byte integers, or the reverse). Note,
  however, that incompatible datatype implementation may prevent this.
  
  If you cannot use an Oracle Net database link and the data file must
  be accessed by SQL*Loader running on the target system, then it is
  advisable to use only the portable SQL*Loader datatypes (for example,
  CHAR, DATE, VARCHARC, and numeric EXTERNAL).