Local recording (recording to a device) is not possible on iOS and android devices. To record Zoom lectures on a mobile device, you will have to request a Zoom Cloud Recording license from ETS here. This will allow you to record and directly upload your session to your Zoom account on the web. You will be able to access your recording through the Zoom web portal once your meeting has concluded: https://ucsb.zoom.us/recording .
For detailed instructions regarding Cloud Recording on mobile devices, refer to the following page from Zoom’s Help Center: https://support.zoom.us/hc/en-us/articles/203066759-Recording-on-iOS-and-Android