The template of UAI 2018 can be downloaded: http://auai.org/uai2018/submissionInstructions.php. The first sentence of "LaTeX 2.09 sample file"
and "LaTeX 2e sample file" is documentstyle and documentclass, respectively. If you want to use "\usepackage", you have to use
"LaTeX 2e sample file" rather than "LaTeX 2.09 sample file".