From the Hilton Honors terms and conditions:
https://www.hilton.com/en/hilton-honors/terms/
A new Hilton Honors Member may purchase Points 30 days after enrollment if the account reflects activity including a stay (as defined herein) or earning Hilton Honors Points through a
Hilton Hilton Honors Marketing Partner. After 90 days of enrollment, a new Member is eligible to purchase Points regardless of their account activity.