"Playback" option is only for playing recordings stored in camera SD card or HDD inside NVR. If you use a camera with no SD card (e.g. RLC-410), then "Playback" option won't be used in your case. For more info, please refer to: https://reolink.com/faq/remotely-playback-record-on-via-reolink-client-from-sd-card-or-nvr/
For recordings stored on your PC HDD or external HDD, you can go to the folder and find the video files, then double-click to open with media players.