Symantec™ Encryption Desktop (Formally PGP Desktop) includes functionality to use user credentials to enroll the user behind the scenes, thus eliminating any user intervention during the enrollment process. This feature is called Invisible Silent Enrollment (also sometimes called Super Silent Enrollment).
Invisible Silent Enrollment incorporates using install strings with msiexec. If another install string is being used “PGP_SILENT_FORCE_LDAP=1 this will negate the PGP_INSTALL_DISABLESSOENROLL=0, thus causing the enrollment credential pop-up.
Remove the PGP_SILENT_FORCE_LDAP=1 from the install string to resolve this.