Sometimes when you load file in LSMW (Read Data step) system throws an error with a code without any human explanation. To understand this I advise you to look at this ABAP code from the standard LSMW program. Having this function module return codes (the same time error codes for LSMW) it would be easier to understand what’s wrong. Here is the list of LSMW errors when you work with text files. This step occurs just before you run convert data in legacy workbench. These return codes represent errors SAP shows us during file read step.

program /SAPDMC/SAP_LSMW_READ_FORMS:
CALL FUNCTION ‘GUI_UPLOAD’
EXPORTING
filename                      = l_filename
filetype                      = l_filetype
codepage                      = l_codepage
*     HAS_FIELD_SEPARATOR           = ‘ ‘
*     HEADER_LENGTH                 = 0
*     READ_BY_LINE                  = ‘X’
*   IMPORTING
*     FILELENGTH                    =
*     HEADER                        =
TABLES
data_tab                      = “l_upload_table”
EXCEPTIONS
file_open_error               = 1
file_read_error               = 2
no_batch                      = 3
gui_refuse_filetransfer       = 4
invalid_type                  = 5
no_authority                  = 6
unknown_error                 = 7
bad_data_format               = 8
header_not_allowed            = 9
separator_not_allowed         = 10
header_too_long               = 11
unknown_dp_error              = 12
access_denied                 = 13
dp_out_of_memory              = 14
disk_full                     = 15
dp_timeout                    = 16
OTHERS                        = 17