You have a first gen single bay My Cloud based on the firmware (v04.05.00-315) listed in that screen capture.
One can also compare what their My Cloud looks like to the images listed on the WD Support site:
https://support.wdc.com/cat_products.aspx?ID=1
One can also check the P/N number on the bottom of their My Cloud and perform an internet search for it.
https://community.wd.com/t/my-cloud-gen1-or-gen2/174179/3
The single bay My Cloud units do not officially support adding the Plex Server app. Unofficially it may be possible if one has a 2nd gen single bay My Cloud.
https://community.wd.com/t/wd-mycloud-gen2-enable-apps-install-tab-apps/177885/
