I know of some of the reasons for getting this message, and some I don’t. Let’s break this message into two parts to explain it better:
"MyCloud is offline . . . " This simply means you cannot access the My Cloud with your device (iPhone, iPad, etc.) while running My Cloud app for some reason. It actually may not be “offline” ,so check it with your PC or other way to access the My Cloud other than with the app.
". . . local datas are available" this means the My Cloud app can access and play files you have downloaded with the My Cloud app to your phone, etc. and are saved in the “Downloads” section of the app. Makes sense.
So, the problem is likely within the first half of the statement. i.e., you cannot access the My Cloud device with your phone. So, be sure you are connected to your wireless network if you get this message, and figure out why if you are not connected. No signal? Weak signal, etc. Did you simply try the access too soon before My Cloud came out of sleep mode? Whatever the cause may be.
One reason I tend to get this message sometimes is because I have two WD devices linked in My Cloud app (the My Cloud NAS and the Wireless Passport) and if I had last used the Wireless passport and then want to upload photos to My Cloud, I have to remember to re-select the correct device to access from the 3-bar menu. I see I was still connected to wireless passport (even if it was now turned off) so I have to now select My Cloud device again.
That’s the issue, in a nutshell.