If you do a search, using the magnifying glass icon upper right, through this subforum you’ll see this issue is well discussed. If you have a single bay My Cloud the official answer is you cannot install Plex. If you note the bottom of the link you posted it lists the products installing the Plex app applies too and the single bay My Cloud is not listed.
Applied Products:
My Cloud EX2
My Cloud EX2100
My Cloud EX4
My Cloud EX2 Ultra
My Cloud EX4100
My Cloud PR2100
My Cloud Mirror
My Cloud DL2100
My Cloud PR4100
My Cloud Mirror (Gen 2)
My Cloud DL4100
Now unofficially there may be ways to install Plex to certain firmware versions of the single bay My Cloud but it is NOT supported by WD and generally the transcoding feature of Plex doesn’t work on most if not all the My Cloud units due to the lower powered processor used by the My Cloud hardware.
https://community.wd.com/t/using-plex-media-server-with-my-cloud/94429/13
There is also a potential way to unofficially hack the v2.x single bay My Cloud to allow for installing certain third party apps that the more expensive My Cloud units support.
https://community.wd.com/t/wd-mycloud-gen2-enable-apps-install-tab-apps/177885
