File 'utf8x.def' not found error in TeXstudio on Ubuntu 16.04

If you installed with apt-get, use sudo apt-get install texlive-latex-extra

If you happened to install otherwise such as BasicTeX, Windows, or a manual install on Linux, you can use tlmgr and run sudo tlmgr install ucs.

Have you tried to see if utf8x is installed?

You could try apt-file -x search '/ucs.sty$'

According to CTAN this is included in texlive, so install this if not already installed.