How to correct Message Code HEL 0510 ?

how can I correct this warning ?

Programa: YY_TEST Línea: 8 [Prio 3]
Do not declare fields and field symbols (GLB_TXT) globally.
Deactivatable using pragma ##NEEDED. Message Code HEL 0510

this is the ABAP code -->

REPORT yy_test.

DATA glb_txt TYPE string.

START-OF-SELECTION.
glb_txt = 'HELLO WORLD !'.

END-OF-SELECTION.
WRITE / glb_txt.