You need latest Pulse Secure iOS client(7.0.0) for iOS12. This is currently in Beta phase and please send a mail to email@example.com for Beta enrollment
Following article mentions couple of workarounds to resolve this issue. Please check if this helps.
The workarounds doesn't work. The certificates are no longer visible. Manually importing a certificate into the app ends with the following error. "Error adding certificate to Pulse Secure Keychhain. Status: -34018"