Hello Alita,
Please, adjust the call of GET_COMPONENT_LIST by passing not the internal table for an import parameter FIELDNAME, but the char-like variable or literal holding the name of the internal table for which you would like to retrieve its components. The simplest way here would be to put internal table's name in quatation marks:
CALL FUNCTION 'GET_COMPONENT_LIST'
EXPORTING
program = sy-repid
fieldname = 'itab'
TABLES
components = lt_comp.
Regards,
Vadim