The {{ update_preferences }} merge tag is used in Newsletter Glue to render the Profile update form link.
Before being able to use the {{ update_preferences }} merge tag, you are required to create a Profile update form in your Brevo account.
NOTE: Without the Profile update form, the merge tag {{ update_preferences }} will not render and instead be replaced with an empty string.
In your Brevo account, please go to Contacts > Forms then click the Profile update tab like so:
In the Profile update tab, click Create profile update form.
For more information about how to create a Profile update form, please check this link from the official Brevo documentation: https://help.brevo.com/hc/en-us/articles/360003644360-Update-your-contacts-details-and-preferences-profile-update-form.
After you create the Profile update form, you can now safely use the {{ update_preferences }} merge tag in Newsletter Glue.