The purpose of this study is to assess the safety and effectiveness of the investigational drug ZEN003694 in people with advanced squamous cell lung cancer that continues to grow despite treatment and contains multiple copies of a gene called NSD3. ZEN003694 blocks a group of proteins called BET, which may counteract the effect of NSD3 on tumor growth. Blocking these proteins may slow or stop the growth of lung cancer. ZEN003694 is taken orally (by mouth).