If users block third-party cookies (browser settings) they won't be able to access the Gift Card module in admin, there's a cookie set in another VTEX domain.

After blocking third-party cookies is possible to simulate the issue mentioned above, you can also inspect the page (developer tools) and go to Network tab (search for Vale.aspx), there will be a 302 status code.

Whitelist third-party cookies present in myvtex.com domain.