I'm unable to retrieve the values for DFKKBPTAXNUM-TAXTYPE and DFKKBPTAXNUM-TAXNUMXL

I'm incapable to retrieve the values for DFKKBPTAXNUM-TAXTYPE and DFKKBPTAXNUM-TAXNUMXL, it is returning each fields arsenic blanck utilizing the code:

ls_dynfields-fieldname = 'DFKKBPTAXNUM-TAXTYPE'.

APPEND ls_dynfields TO dynfields.

ls_dynfields-fieldname = 'DFKKBPTAXNUM-TAXNUMXL'.

APPEND ls_dynfields TO dynfields.

CALL FUNCTION 'DYNP_VALUES_READ'

EXPORTING

dyname = 'SAPLBUPA_BUTX_DIALOG'

dynumb = '0100'

translate_to_upper = 'X'

DETERMINE_LOOP_INDEX = 'X'

SEARCH_OWN_SUBSCREENS_FIRST = 'X'

SEARCHPATH_OF_SUBSCREEN_AREAS = 'X'

TABLES

dynpfields = dynfields

EXCEPTIONS

invalid_abapworkarea = 1

invalid_dynprofield = 2

invalid_dynproname = 3

invalid_dynpronummer = 4

invalid_request = 5

no_fielddescription = 6

invalid_parameter = 7

undefind_error = 8

double_conversion = 9

stepl_not_found = 10 others = 11.