This warning appeared inexplicably after I mounted two previously inactive disks as RAID 1, assigned a UUID to it, and later there was a kernel update.
To fix the warning, the first thing to do is
$ lsblk -f
to obtain the UUID value from the line "/".
This UUID is then entered in two files. In the file etc/fstab in the line containing the root "/", e.g:
"/dev/sda3 / ext4 defaults 0 0". The part "dev/sda3" is replaced by "UUID=value":
UUID=cb2f354f-9e7a-44f8-8a01-2ef1e0b66c31 / ext4 defaults 0 0
In the file /etc/initramfs-tools/conf.d/resume the UUID is entered in this form:
Then the following command:
$ update-initramfs -u -k all
The warning should have disappeared and a reboot should be running.