Taking it as you are using KDE Plasma, have a look at the following link/thread:
Perhaps also the output of the following command could be useful if the above won’t work for you.
With the disk you wanted to be mounted at boot attached:
lsblk -fs
and also the content of your /etc/fstab:
cat /etc/fstab
Would be useful to see the exact line you had added to /etc/fstab.