Ubuntu 18.04 can't write on NTFS partition
your windows is putting the "read-only" flag on all detectable ntfs partitions everytime it shuts down or reboots.
If you don't want it to do that, boot to your windows, open up a administator powershell (to do that find powershell in start, right click on it, then select "run as Administrator").
then type :
powercfg -h off