@domnantas: You need to enable Private Messaging. This is in the "Edit Options" of the control panel. If this isn't there, it has to be added somehow. You can try posting in
https://www.mobileread.com/forums/sh...d.php?t=319819 to ask it to be enabled, or contact a moderator.
You should also try a factory reset using the instructions in
https://help.kobo.com/hc/en-us/articles/360017605274. But, it is likely the internal SD card is failing and needs to be replaced.