Captive Portal along with a Wi-Fi password - is it possible and how?

Question

WiFi

There is a small WiFi network with Cisco equipment (now Cisco Mobility Express virtual WLC, I'm trying to set up an iron WLC), two WLANs, one working, the second guest, but with a password (WPA2 PSK). The client, whose Wi-Fi, wants to set up a captive portal on the guest Wi-Fi, the controller, when trying to turn it on, issues Invalid WLAN Policy (WPA / WPA2 SPLASHPG-WEB-REDIR PSK), such as it is impossible. I don't understand what prevents you from turning on the captive portal in the "show page" mode on Wi-Fi with a password. The question is how to enable both settings at once? And if not, why not?