Under Ubuntu 16.04, building a PDF under TeXstudio worked. Under Kubuntu 17.10 (with pdfTeX 3.14159265-2.6-1.40.18 (TeX Live 2017/Debian)), running the same input files resulted in:
! LaTeX Error: File `textgreek.sty' not found.
These seemed like an error with reported in Feb. 2015, as “texlive-latex-extra: missing dependency – textgreek.sty requires lgrenc.def (texlive-lang-greek)“, but somehow this error was different.
A post at tex.stackexchange on “How do I install an individual package on a Linux system?” suggested instead …
Find and install the package through your package manager. [….]
File Packages /usr/share/doc/texlive-doc/latex/textgreek/textgreek.pdf texlive-science-doc /usr/share/texlive/texmf-dist/tex/latex/textgreek/textgreek.sty texlive-science
So, textgreek.sty is available via apt or Synaptic, when installed with texlive-science.