The lack of an on/off button within the Dashboard is a well known issue discussed in several prior threads. the solution is to either use the WD Access program or issue the shutdown command via SSH.
https://community.wd.com/t/how-i-turn-off/159860
https://community.wd.com/t/how-to-safely-shutdown-mycloud/155265
https://community.wd.com/t/safe-shutdown/147307
https://community.wd.com/t/cannot-find-power-off-button-in-dashboard-v2-11-133/151868
