Ubuntu Server 16.04 hard disk space problem.

Find what is using all the space and remove it.

I'd use df -h then du -sh /path/to/hog directory - over and over and over moving deeper and deeper until the hogging files are found.

BTW, posting an image here is not so nice to people with limited bandwidth. Posting a link to the image would be kinder.

I may be a little fuzzy, but that looks like Windwos to me.