Been having this problem for a month. Took Hilton a week to respond via email. William (forum ambassador) is involved. So far, the response from email customer care are… underwhelming.

Latest response from Hilton says to:

• Clear browser cache and cookies, Try an alternative browser.
• In order to redeem a claim code, the person receiving the gift must already have a Hilton Honors account created
• Existing Members should first verify account access by logging into their Honors account
• If you are unable to log in, it is recommended to reset the password to access the Honors account before proceeding
• Do not use any autofill information when logging into the Honors account, if so, delete and manually enter details
• Do not use the copy and paste option when entering the claim code, this will prevent logging into the account
• Last resort, try using the website from a computer or laptop rather than a phone